Quenchable Fire never seems to do the followup damage, regardless of whether you pay for it or not. I've tried testing it speciffically and reproduced it easily - give the AI appropriate mana and the card in hand and then don't pay to make the aftereffect go away.

From what it looks to me the aftereffect is being automatically cleared in the wrong players turn (opponents) instead of lasting until the casters upkeep when it's supposed to do the damage.

Could be something else, I'm just trying to be helpful.