Supposedly this is fixable on our end. Even if it shouldn't have to be. https://github.com/Beethoven92/BetterEndForge/blob/master/src/main/java/mod/beethoven92/betterendforge/mixin/BiomeColorsMixin.java#L31
Asek3/Rubidium#448

I'd advise against using the BetterEnd solution without doing more research first; I am not sure that getBlockState can safely be called on the client world from another thread (this is why WorldSlice exists in the first place).

also, relevant issue Asek3/Rubidium#468

Might just PR a fix to Rubidium myself when I have the time.

Opened a PR here instead of trying to find a workaround on Aether's end: Asek3/Rubidium#475