If you try to spawn a slabfish when Alex's Caves is installed, the game won't actually spawn a slabfish. I went through the entire entity constructor on debug mode to try to figure out a way to solve the issue, and I did! Yay!

Alex's Caves has THIS entity mixin:

Which calls LivingEntity.getActiveEffects(). getActiveEffects() returns LivingEntity.activeEffects.values(). LivingEntity.activeEffects is null until it is initialized in the LivingEntity constructor, so a null pointer is thrown. In other words, this mixin makes LivingEntity.isInWater() throw a null pointer if it is called before the LivingEntity constructor is called.

The Entity constructor, which is called before the LivingEntity constructor, calls entity.getEyeHeight().

Slabfishes uses their own getEyeHeight() implementation, which calls isInWater().

This causes a crash.