Dreadful dirt spawns baby zombies that are immobile and cannot be attacked or killed.
just like the jpg showed

it happens in modpack: FTB oceanblock 2

I haven't played oceanblock 2, but i would suspect there is a script or another mod taking the AI away from the baby zombies.
MGU spawns mobs normally, and all the same, there isn't any special treatment of one mob or another, and i cannot reproduce this in a normal environment.