Skeleton Horses despawning, despite being name tagged.

One horse was inside my base, the other outside but had a roof over it.
One was leashed to a fence post, one wasn't.
They both despawned.

They have both been in my world long term (over a year). Upon logging in recently, they had both despawned.

No other mobs in my world have despawned, just the skeleton horses.

Current results:
Named skeleton horses are despawning.

Expected results:
When a mob is named with a name tag, it should not despawn.