Hello: I am a bit confused about the power venting. Usually I’d expect to see the fan continue to run when the burner fires. Also, they can be noisy, so should be easy to hear operating, even if at a lower speed.
In the owner’s book, there should be a description of the operating sequence for the furnace.
The other thing was to check draft while any exhaust fans in the house are running. This could give you more time backdrafting than the furnace alone seems to be creating.
If it’s a wild goose chase, at least it’s an interesting one 😎