A stuck open ash dump door would actually make your fire burn hotter if anything, since that's where your air comes from on a rivet forge. The dump door is basically the bottom air gate, so having it cracked open just feeds the fire from below. What usually kills a forge is ash building up and blocking that same hole, or a clinker choking the air flow. Cold air sneaking in through a gap around the door, not the door itself being open, sounds more like what got you. Either way glad you tracked it down, those little gremlins will drive you nuts.