What?
In V3.0 Table 11 Metadata Attributes all classes and the fields that are not available in metadata representation are listed.
This also includes AssetAdministrationShell.
Value-Only was designed as the opposite of Metadata: just do not display metadata information.
In the API for AAS only $reference is supported but neither $value nor $metadata.
How?
Either add $value and $metadata for AAS as well in AssetAdministrationShellServiceSpecification
or just add a note:
"Note: Although metadata and value-only representations of an Asset Administration Shell are possible they are not supported up to now."