Odd rendering with `anvil` on 1.20.1 modded world

[rjp]

For some reason, anvil is rendering my modded 1.20.1 world strangely (see below.) It's ignored a bunch of grass blocks etc. (which seem to be valid colours in the palette, also below) and rendered a variety of different levels below the ground (I could understand if it had gone for, say, a consistent Y=0 but there's bits from Y=46 etc.)

Doesn't seem to be a problem with fastnbt in general - I can use that in my own code to look at blocks in the world just fine.

anvil output:

minutor version of approximately the same area (the magenta blocks are from mods and minutor has no palette for them):

Palette used: palette.tar.gz

Region file (because I can't attach it): https://rjp.is/r.-1.0.mca

Mod list (in case it helps):
mods.txt

Command used: ./target/release/anvil tiles $HOME/apriloons2025 --out aptiles --palette $HOME/palette.tar.gz