Axolotls always despawn

I'm having an issue with the new Axolotls. I play on a realm and I have collected three Axolotls, safely kept in a large tank of water with perfect lighting, with name tags and no drowned spawning. A while after leaving, I come back to the Axolotls disappearing. I know the people in the realm aren't killing them as my base is a well hidden area and I believe this is just another annoying bug because this just keeps happening every time.