Zombies taking ages to die whilst burning

I was at a village with my puppy (in creative)  when I spotted some zombies outside a villagers house. I waited in a church where there were no mobs outside the door until day but when day came the zombies weren't burning at first. About a minute later, they all started to burn. I waited a few more minutes and they still hadn't burnt to death. My puppy was also angry for some reason because I left it as a wild one and put it on a lead. There were no sheep or skeletons or rabbits around so it was angry for no reason.

I hope you can fix this soon.

Kind Regards,

DogGirl108

Edit: I don't know if this is happening to anyone else.

Edit 2: This also happens to me on Survival mode