I had 3 sniffers in an enclosure, each named using a nametag. Two were from eggs I got from suspicious sand and the third was from an egg resulting from breeding, and the resulting sniffer is the one that disappeared, if relevant. After it hatched I named it and fed it, then went to do other tasks. When I logged back into Minecraft today the third sniffer had disappeared. The other two are still in place, and I tested the enclosure, it couldn't have escaped. Why did this mob disappear?