[MC-91290] Skeletons with no AI spawning naturally

To reproduce, follow the steps taken in this video: https://www.youtube.com/watch?v=UgiTzg2jr_A

Mod Notice

When confirming this bug for a new snapshot please only do so if you have experienced the problem in a world created in that snapshot. Thank you.

New survival game started at 15w43b.
90% of the time spent in a village in a plains biome.
1 or more skeletons with no AI would spawn each minecraft day around the village (or just player?).
These skeletons can be hit once and no more but still remain.
These skeletons if enough keep spawning in each minecraft day will prevent other mobs from spawning.
Saving and reloading will remove these skeletons however they continue to keep respawning each minecraft day.
Not related to Skeleton Traps.

All relevant reports (~~MC-91461~~, ~~MC-89923~~ and this one) suggest this issue is related to the Passengers tag:

The same issue occurs on occasion (thought very rarely) with baby zombies.

Others suggest the issue is related to despawning.

Removed references to Skeleton Trap since no AI skeletons occur without them.