Axolotls despawning after some time

After making a underground enclosure for axolotls, me and my brothers went to grab axolotls with a bucket. We put them in the big enclosure, and after probably like 5-10 Minecraft days, we came home to find them all gone. They had up to 3 block deep water, and some of them even had name tags. We kept retrying to put more axolotls in the enclosure, but they just keep despawning and its ruined a big plan to make a huge habitat for axolotls. It's also a fact that bucketed axolotls wont despawn when removed from bucket, but that turns out to be false. I am assuming this is a bug with the axolotls considering they also despawn with nametags, which nametags are supposed to stop things from despawning.