Today on a friend's private realm I noticed that two of my named foxes were gone from their enclosure. After further inspection I found that 7 named mobs had vanished: 2 foxes, 3 turtles, 1 horse with armor, 1 jeb_ sheep and 1 Cave Spider. This interesting as I recently brought in 6 new named mobs into another while naming another unnamed mob that was already there: 3 dolphins, 1 skeleton, 1 stray, 1 polar bear, and 1 tamed cat. My theory is that if a certain number of named mobs exist in the same area, adding anymore to the area will force some to despawn. That's the only thing that makes any sense.