When naming a Mob, hostile, neutral, or passive, it is supposed to prevent that mob from despawning under any circumstance. However, it seems that this no longer applies to hostile mobs, and various neutral mobs (Zombie Pigman, Polar Bears, Village Iron Golems, etc.)

This can break several different things, such as minigames, farms, decorations, or redstone machines, and is ultimately a shame if this feature was removed. If this was an intentional change, please bring it back, and if it truly is a bug, I hope you can find how to resolve this issue.

And this is not single entities despawning with names, or them randomly despawning.
Each and every entity that is named that has the ability to despawn WILL despawn.