Axolotls despawn randomly. I captured one in a bucket making it a bucket of axolotl and name tagged it and everything. I had a pen on a survival world for him. There was a little pond with no current so he couldnt of drowned. His name was Charlie. He disappeared without a trace. I went to reddit and saw that numerous amounts of people were having the same exact problem. Many said the same as I, captured them in a bucket, name tagged them, had the axolotl by water, and came back to find them gone.