The bug

If a ghast or vex is ridden by a mob, it will only move for the first few seconds after being summoned. After that, it will remain frozen it place, not moving at all.

How to reproduce

1. Summon a ghast or vex ridden by a mob:

   /summon ghast ~ ~5 ~ {Passengers:[{id:cow}]}

   /summon vex ~ ~2 ~ {Passengers:[{id:cow}]}

   → Intitially, the ghast/vex flies around
   → After a few seconds, the ghast/vex stops moving (permanently)