The bug

If you summon a skeleton horse with PersistenceRequired, it will despawn once it's SkeletonTrapTime reaches 18000 just like a skeleton horse without PersistenceRequired.

How to reproduce

1. Switch to Spectator mode:

   /gamemode spectator

2. Spawn a skeleton trap horse with the PersistenceRequired tag and a SkeletonTrapTime of 17900:

   /summon skeleton_horse ~ ~ ~ {SkeletonTrap:1,PersistenceRequired:1,SkeletonTrapTime:17900}

   → The skeleton trap horse despawns after 5 seconds