Me and others on my Realms server have made Axolotl habitats, named them with name tags too and made sure they can't escape. But once everyone leaves the realms for a few hours and then someone comes back online most or all of them have despawned. We've kept them in buckets for now to save them but we still want to be able to actually place them in the realm. This seemed like a bug so I thought I'd bring it to someone's attention. 
Thank you.