I have been collecting axolotls and putting them in pond, when I put more than 3 or 4, they disappear without trace. No death message, no subtitles saying they are taking damage or died. I recently put a new axolotl (named-tagged), and in matter of seconds, when I left them out of my sight, they disappeared.

How to reproduce bug (for what I've tried):

1. Place various axolotls (more than 3-4) in same chunk.
2. Leave them out of your sight.
3. They disappear.

Note:

This also seems to only affect single player worlds, as I've tried to replicate this on creative.