Holy Man Campaign - Villagers attack priest on Normal, but not on Hard setting

I found several people with similar issues when googling for this problem, but no explanation.

I'm having a hard time playing through the Holy Man campaign, so I tried looking for a gameplay video on youtube and I noticed that villagers behaved differently on the video than they do in my game.

When I try to convert a villager, they'll immediately attack the priest, often taking around half his life before the conversion is complete. If I try to convert a second villager the priest usually dies.

On all playthrough videos I saw, the villagers ignore the priest completely and they can convert several enemies with ease.

Now when I googled about it, I found an user commenting that it only happened on the lower three difficulty settings, while on hard the villagers ignore the priest, making the campaign much easier on harder settings. I tried it here and that's exactly what happened.

Anyone knows why this happens?