I have a Minecraft PE world where I had built a research facility and I had decided to add a pet room. I put some wolves, cats, and foxes each in a separate room that connected to a gathering spot, and each room didn't have doors so the pets could mingle. I had named all of them, even the foxes, and they were all trusted. I'm not sure when this occurred because I just noticed it today but my favorite fox Tinder wasn't in the enclosure. Keep in mind, it was an actual room with walls, not a fence. The only way to get out was through an iron door with a lever. I decided to do some research and I found he couldn't despawn due to the name tag and trust, though he might've been in an unloaded chunk which would have caused it. Quite a few others have reported disappearing animals when they were fed and named, I'm not sure if it's related.