The “Wildfire” ability used by Hellspawn mobs leaves an invisible area of fire on the ground after the mob is killed.
Sometimes it’s visible, sometimes not. IMHO, it’s the same bug as the one that causes the CF graphic to vanish sometimes. It’s hard to tell due to the dim lighting in the scenario, but I think the filth puddles vanish sometimes, too.
I get this too. Took a while to figure out why I was getting so many stacks of burning, and they can kill you real fast too.