No Doubt a real mind bender. After reading the entire thread and now knowing the exposure settings and that a Strobe was fired. I would have to agree that this is a photo anomaly.
I sat here bending my brain looking for reasons to validate my 2cent theory and I could not. The single thing that changed my thinking is the fact there is no smoke visible. The reason there is no smoke visible I think is because there was no light available to show it. And as previously argued the strobe froze the action right before hammer strike and the only light available for exposure afterwards was the glow of combustion. The smoke was there but the with light available the camera could not record it.
The question is not not one of firearm physics and engineereing but rather camera physics and engineering.