The bug

Hostile mob without bow, such as zombies, creepers, when they ride a horse, sometimes they can't pathfind well.

How to reproduce

1. Run

   /summon horse ~ ~ ~ {Passengers:[{id:"husk"}],Tame:true}

2. Run

   /summon villager
   

   → Notice the horseman and horse can't surmount some very easy obstacles, even the their hitbox is permissible