I noticed that zombie villagers can despawn after you gave them the golden apple to turn them into normal villagers again.
You can lose golden apples due to that and since they're somewhat valuable I think it shouldn't be possible for them to despawn after you started curing them.
Environment
Put your operating system (Windows 7, Windows XP, OSX) and Java version if you know it here
Can't reproduce in 16w42a: they didn't despawn when I went more then 128 blocks away