Was messing around in a 1.21 dev environment for planetarium and came across this: https://mclo.gs/X1UNhbq

Yeah so this is a weird one - I think the fix for this is QuiltMC/quilt-loader#432, as I think remapping is the only reason that mods might get split up.

oh ok

Quilt simply refuses to load on 1.21.1.

Correct, please use 1.21 or loader overrides to get the 1.21 version to work on 1.21.1

Correct, please use 1.21 or loader overrides to get the 1.21 version to work on 1.21.1

If the mod can't load on the version it requires, then please remove these unloadable mod versions.

They are not marked as compatible.