I've had puff backs with my AHS several times, some more robust than others. The real cause remains elusive. Clearly it's just the right combination of coal gases, ignition source and combustion air. The two times I observed them it occurred when the combustion blower had be running and then stopped. Clearly there was a good ignition source then. The Bureau of Mines report describes puff backs occurring with buckwheat coal and a fix was to provide additional air by drilling a hole in the damper door. I have observed more puff backs seem to occur with fresh coal. Coal that was mined in the last few weeks. I've read reports that all coal, including Anthracite oxidizes, i.e. it combines with oxygen in the air reducing it's Btu content. For Anthracite coal it is a small effect but it does occur. The effect would be greatest shortly after the coal is mined. So, it kind of explains my observation, I can get puff backs with fresh coal. I don't seem to get them with coal that's been uncovered and sits outside in the weather.
My AHS coalgun has a flapper door with a bar across it. I suspect the bar was an improvement to restrict the flapper door movement during a puff back. I've see photos of newer coalguns which have sort of a hood over the flapper door with an apparent opening on the bottom the hood. I guess it's an attempt to get a chimney effect and clear out any combustion gases.
My general recommendation is to join the flue pipe securely with three screws, tape the joints with high temperature aluminum faced tape and securely support the pipes. Same for the barometric damper. For me the cause still is not fully explained and I can not cause it to happen on purpose. I've tried. My frequency of occurrence has been 3-4 times a heating season. It doesn't seem to occur during my summertime heating of domestic hot water only.