When you have e.g. a renamed pig and it gets struck by lightning, it gets turned into a Pigman which still got the name of the pig, but its PersistenceRequired tag which was set to 1b by renaming is gone (= 0b), which results in despawning of this mob.
The same goes for e.g. a Villager turning into a Witch, the Witch will keep the Villagers' name, but will despawn/her PersistenceRequired tag will not be set automatically to 1b.
Now, at least with upcoming raids in Minecraft 1.14, and with the possibility that a previously Villager who gut turned into a Villager Zombie and then cured again to a Villager retains his old trades, this seems to be a logical conclusion that a "turned" mob should stay persistent, also considering that they keep the nametag-name after all.
This is not a suggestion, but I consider this a bug, however, it is completely up to Mojang to decide whether or not this is not a bug, but WaI.
Pictures of the PersistenceRequired tag etc. in this post, video by @unknown here: https://www.youtube.com/watch?v=UIJYFFwi2NA
Sorry if I overlooked a bugpost for this issue, I looked around and couldn't find it, which I find puzzling, as I'm quite sure that it must have been mentioned already, but probably there are again no tags in the bugpost, or none with which I could find them.
Linked issues
Attachments
Comments

This is covered by the more general report MC-88967

Okay, thank you very much for the link, @unknown!
Would it maybe be possible to update the labels so it can be found with other search terms?
I seem to always suck so badly to find bugposts, apparently mainly because the labels are not inserted with which I would think I would usually find them.

The labels are not actively maintained, only some of us add them sometimes (I think) and remove malformed ones.
Maybe this reddit comment helps you with searching.

I already search for any possible resolutions as well as createdDate and such 🙂
But my search terms, the words themselves, just don't always seem to be the right ones.
But thanks!
I'll rather post a duplicate then again in the future, just to be safe, and apologize in advance for any additional work that may cause for all of you.
I do search, I just don't always find them 🙂
Can confirm: https://www.youtube.com/watch?v=UIJYFFwi2NA