Axolotl disapear randomly even when name tagged and in an enclosure

I have caught and homed half a dozen axolotl or more and made it so they could not leave an enclosure.  I name tagged one after 5 despawned.  The name tagged stayed a while longer until it disappeared also. None of the turtle in the aquarium disapeared. No other animals disapeared either.  It is always axolotl.