When some Mobs spawn, they just freeze. You can try to punch them, but it won’t make a difference. This only happens once in a while, and when a hostile mob is frozen, it takes a rejoin or a quick difficulty switch to get it working again. This also happens when Villagers sleep. When they sleep, they can be pushed around, and they can clip through most blocks. I first found this bug in 1.16.100, but please fix it in the upcoming 1.16.210.50 update!