Steps to reproduce:

1. Set the difficulty to peaceful (/difficulty 0)
2. Summon an iron golem (/summon VillagerGolem)
3. Make sure you're in Survival mode (/gamemode 0)
4. Attack the golem
5. The golem will repeatedly swing its arms at you, but you won't receive any damage or knockback

I would expect the golem to either deal damage or not swing its arms at all.