Last year, after limping in six hours late to cone 10 with one burner out because of back burning in the tube I disassembled the unit and behind the orfice, supply side, I found a sticky very miniture spider egg sack partially obstructing the orfice. Imagine a spider small enough to crawl through a propane orfice. My gass supplier/tech tells me other corrosion matter or debris can and does cause the same burning in the tubes.
happy future fireings, your problem may be this simple to solve.
still peering over the edge, reverently taking an irreverent look at everything.