This is not only happening to me, but also to a few others in reddit. Below is their explanations. Someone did say about this, but it was filed as two issues. So what happened was me and a couple of friends had an axolotl enclosure. We grabbed four axolotls and four nametags and used them. After a few days passed, we checked on the axolotls and saw all but one were gone. Nothing said in the chat that signified that they were dead. There was no other water resource for them to reach. The enclosure was even surrounded by fences. We searched for them but couldn't find them. We then assumed a friend stole them (we did apologize). We did find one of them just today and took it back into the enclosure, but after a few minutes it was gone again. We made sure the enclosure was secure and that the axolotls wouldn't escape. So we were confused. Until we searched up "Disappearing Axolotls in Minecraft" and turns out we weren't the only ones that had this problem. This was happening for a month. We believe its a bug and hopefully it gets fixed!