Axolotls despawning though named and on leashes.

I've had several axolotls that have been named, the first four were not on leashes and I figured when I came back that they had died or wandered off, but now it's happened again, though the axolotl was named and leashed.

I get emotionally attached to them and it makes me very sad to have them despawn for what appears to be no reason at all.