Name tagged mobs despawn

How to reproduce:

• make an area where the mobs cant flee or die
• spawn 2 hostile mobs (I tested with Zombie Pigman)
• name only one of them with a name tag
• go a bit farther and do something else/wait (I waited 20 minutes)
• both mobs will despawn, not only the one not given a name

Update: still happens on 0.15.10