Hello: Backdrafting is simply flue gasses/air coming down the vent pipe and spilling out, rather than going up and out the vent as they should. I’m sure a more technical definition exists, but this one gives the idea.
The vent height of six or seven feet is not going to create a very strong draft. If there is any horizontal portion to it, the draft will be weaker still. If the top of the vent is not sufficiently far above the roof, wind could cause problems with the draft.
Whether or not there is a faulty burner is a question for the manufacturer. There have been rare burner problems in the past. Giving them model and serial numbers will let you know if it is a recognized problem. If one wanted to get really scientific, the yellow deposit could be tested by a lab to determine just what it is. But, spending a few minutes and watching the flame on a breezy day will likely give a very good idea how well the burner is performing.