The bug

When an iron golem / a VillagerGolem kills a monster it is riding, its AI gets broken and it only turns around the whole time.

How to reproduce

Use the following command and let the VillagerGolem kill the Zombie it is riding

/summon zombie ~ ~ ~ {ArmorItems:[{},{},{},{id:"white_wool",Count:1b}],Passengers:[{id:"iron_golem"}]}