Earlier today, I was going out to mine for a decent amount of time, and I had a gold axolotl in a large pool next to my house, and there was no way he could have died, and yet, I come back with some more copper and iron, and he is gone. There was no trace, and I searched all over the pool, and yet there was nothing, which was weird since I didn't know they could be despawned after picking them up with a bucket, but he was gone obviously. I had also found another axolotl, so I just put them in. I kid you not, I paused the game, and it wasn't just opening settings, you would totally pause the game, and no time would pass if you pressed ESC in single-player, unlike multiplayer for obvious reasons, to simply eat lunch, and after maybe 10 minutes, the new one is gone. So somehow, it despawned while my game was paused. I eventually tested something similar in a new creative world, and 3 axolotls would just despawn, whether you name them, move them, or do other things. So somehow, axolotls despawn no matter what you do, and I know that isn't intentional.