Passive and aggressive mobs alike will disappear after being ascribed a nametag. The name will successfully attach to the mob, but the mob will despawn during a relatively short span of inactivity on the Realm server (~6 hours or so). This happens regardless of whether or not they are near any walls to potentially suffocate in.

Example 1: I had 3 polar bears. One had a nametag. I left them to wander freely in a valley that they couldn't scale out of. The named bear disappeared within hours. Up until naming it, I had successfully kept all 3 bears within the enclosed region for weeks.

Example 2: I had a named endermite enclosed in a one block space. It disappears every time within half a day.

Example 3: I applied a nametag to a pig and leashed it to a fence post in an otherwise open field. It is gone.

Example 4: Every single named horse and sheep (>20 of them) that were successfully hanging out in a paddock for months (long before 1.10.1) are gone.

Thanks.