I don't see any ash laying on top of the ties. So if they did dump while turning, they sure cleaned up well. Additionally, the ash pan is about at the middle of the loco-tender length, so the ashes would have landed on the middle bearing if dumpped while turning, so I don't think so.
As close as the tie spacing is on a turntable bridge, I believe we would see quite a bit of tie burning, if dumping ash on the turntable was a regular practice.
All that said, I have no idea what is causing that smoke.