While breeding Axolotls in an attempt to get the blue one I encountered an issue where some Axolotls are unkillable. This is to the point where I can use up the entire durability of a sword and it will not die, staying stuck in the regeneration pose. Some Axolols will turn red and enter the death pose but still won't despawn. This only affects random Axolotls- not all are immortal. I can pick them up in buckets and turning the game on and off again hasn't helped outside of making some not red. 

The pose bug hasn't occurred recently and I didn't get a picture but I hope that this is taken into account. Thanks!