I don’t know how much of a problem this is but I found that Soot Shield is trigger when one does an attack with flaming sword on corpses from slain enemies. Now this could be intentional but I figure that perhaps the talent was meant to only work on live enemies.
To replicate the issue just have the Soot Shield talent and a flame sword with the Battle Wizard Career and make a heavy attack on a bunch of pact-sworn corpses to ignite them.
It occurs entirely consistent for me when I play.