I found a bug...
When you kill someone and it triggers the burning animation, you don't gain experience.
For example, I bought a Hammer of Wrath at the main town. Went to the Sewers, did 2 hits on a Goblin, he died triggering a running burning corpse animation and didn't give me exp.
Edit: From additional testing it seems to also happen with the weapon on slot 9 and the flame thrower hands magic
I didn't test the spells you get from scrolls
Edit2: I took a quick look at the actors folder and it seems that at least Goblin, Undead Corpse and IceMage have bugged burn scripts, there are probably more.