Mobs arbitrarily disappearing

Mobs such as the sniffer, and even hostile mobs that were tagged and named are randomly disappearing. They are not dead, not exposed to the sun; they are locked up in a cell (the hostile mobs), and safe from anything that would otherwise harm them. They still disappear.

What I expected to happen was...:
Mobs, even when named and tagged with a nametag, should exist in my base indefinitely, especially when they are guarded from other mobs and the sun/rain.

What actually happened was...:
Mobs have disappeared entirely, leaving no trace. It was difficult to capture some of these and now I have to do it again with no confidence that the same thing won't happen again.

Steps to Reproduce:
1. Capture a mob
2. House it from the sun/rain and other mobs
3. Tag it with a nametag

4. Go about your business for a few minecraft days and observe