How to get block cloning to work between datasets?

[scaredmushroom]

I'm now fiddling quite a while to get block cloning to work between datasets.
I consulted many similar issues and discussions, but I still can't get it to work.

This is an an encrypted pool. I derive multiple datasets from it.

My data pool:

# zpool get all dpool | grep clon
dpool  bcloneused                     0                              -
dpool  bclonesaved                    0                              -
dpool  bcloneratio                    1.00x                          -
dpool  feature@block_cloning          enabled                        local

Kernel module conf:

#cat /sys/module/zfs/parameters/zfs_bclone_enabled
1

Dataset config diff (omitted personal or sensitive information):

NAME        PROPERTY              VALUE                       |	NAME           PROPERTY              VALUE                   
type                  filesystem                       -	type                  filesystem                       -
used                  3.33T                            -      |	used                  6.09T                            -
available             12.6T                            -	available             12.6T                            -
referenced            3.33T                            -      |	referenced            6.09T                            -
compressratio         1.02x                            -      |	compressratio         1.01x                            -
mounted               yes                              -	mounted               yes                              -
quota                 none                             defaul	quota                 none                             defaul
reservation           none                             defaul	reservation           none                             defaul
recordsize            1M                               inheri	recordsize            1M                               inheri
mountpoint            /mnt/dataset1                    local  |	mountpoint            /mnt/dataset2                    local
sharenfs              off                              defaul	sharenfs              off                              defaul
checksum              on                               defaul	checksum              on                               defaul
compression           on                               inheri	compression           on                               inheri
atime                 off                              inheri	atime                 off                              inheri
devices               on                               defaul	devices               on                               defaul
exec                  on                               defaul	exec                  on                               defaul
setuid                on                               defaul	setuid                on                               defaul
readonly              off                              defaul	readonly              off                              defaul
zoned                 off                              defaul	zoned                 off                              defaul
snapdir               hidden                           defaul	snapdir               hidden                           defaul
aclmode               discard                          defaul	aclmode               discard                          defaul
aclinherit            restricted                       defaul	aclinherit            restricted                       defaul
createtxg             11279508                         -      |	createtxg             837                              -
canmount              on                               defaul	canmount              on                               defaul
xattr                 on                               inheri	xattr                 on                               inheri
copies                1                                defaul	copies                1                                defaul
version               5                                -	version               5                                -
utf8only              off                              -	utf8only              off                              -
normalization         none                             -	normalization         none                             -
casesensitivity       sensitive                        -	casesensitivity       sensitive                        -
vscan                 off                              defaul	vscan                 off                              defaul
nbmand                off                              defaul	nbmand                off                              defaul
sharesmb              off                              defaul	sharesmb              off                              defaul
refquota              none                             defaul	refquota              none                             defaul
refreservation        none                             defaul	refreservation        none                             defaul
primarycache          all                              defaul	primarycache          all                              defaul
secondarycache        all                              defaul	secondarycache        all                              defaul
usedbysnapshots       0B                               -	usedbysnapshots       0B                               -
usedbydataset         3.33T                            -      |	usedbydataset         6.09T                            -
usedbychildren        0B                               -	usedbychildren        0B                               -
usedbyrefreservation  0B                               -	usedbyrefreservation  0B                               -
logbias               latency                          defaul	logbias               latency                          defaul
objsetid              15911                            -      |	objsetid              95                               -
dedup                 off                              defaul	dedup                 off                              defaul
mlslabel              none                             defaul	mlslabel              none                             defaul
sync                  standard                         defaul	sync                  standard                         defaul
dnodesize             legacy                           defaul	dnodesize             legacy                           defaul
refcompressratio      1.02x                            -      |	refcompressratio      1.01x                            -
written               3.33T                            -      |	written               6.09T                            -
logicalused           3.42T                            -      |	logicalused           6.17T                            -
logicalreferenced     3.42T                            -      |	logicalreferenced     6.17T                            -
volmode               default                          defaul	volmode               default                          defaul
filesystem_limit      none                             defaul	filesystem_limit      none                             defaul
snapshot_limit        none                             defaul	snapshot_limit        none                             defaul
filesystem_count      none                             defaul	filesystem_count      none                             defaul
snapshot_count        none                             defaul	snapshot_count        none                             defaul
snapdev               hidden                           defaul	snapdev               hidden                           defaul
acltype               posix                            inheri	acltype               posix                            inheri
context               none                             defaul	context               none                             defaul
fscontext             none                             defaul	fscontext             none                             defaul
defcontext            none                             defaul	defcontext            none                             defaul
rootcontext           none                             defaul	rootcontext           none                             defaul
relatime              on                               defaul	relatime              on                               defaul
redundant_metadata    all                              defaul	redundant_metadata    all                              defaul
overlay               on                               defaul	overlay               on                               defaul
keylocation           none                             defaul	keylocation           none                             defaul
keyformat             raw                              -	keyformat             raw                              -
pbkdf2iters           0                                defaul	pbkdf2iters           0                                defaul
encryptionroot        dpool                            -	encryptionroot        dpool                            -
keystatus             available                        -	keystatus             available                        -
special_small_blocks  0                                defaul	special_small_blocks  0                                defaul
prefetch              all                              defaul	prefetch              all                              defaul
direct                standard                         defaul	direct                standard                         defaul
longname              off                              defaul	longname              off                              defaul

If I do a cp --reflink=auto … on the same dataset it works (shows in bclone statistics), but when I do it between them (I tested with freshly written data from /dev/random), it doesn't.
When running cp --debug I get:
copy offload: yes, reflink: unsupported, sparse detection: no

Does anyone have a clue? :/

Versions:
zfs-2.3.5-1
zfs-kmod-2.3.5-1
kernel 6.12.74