So I play as mage and what point in magic if it is not AOE? So I usually kill enemies in groups, as it is cheaper that way and I found a bug.
And when I was clearing Throatstone mill there was a place with oil on the floor and 2 mages and 1 marauder, I attacked them with fireball staff and chain lightning in other hand, plus oil ignited. And after all was done I got two levelups in row from 12 to 14. There was around 3-4k of experience skipped. This bug is probably easy to repeat. Just in right moment go look for a group of enemies and here it is.