Know you don’t normally go this deep into furnace inspections.
In case anyone is interested here is a video and some photos I took of an Amana furnace that developed flame deviation when the main blower came on. About 1:13 on the time line where you’ll see the flame rise at the hot surface igniter.
Here’s a side view of the removed heat exchanger.
And where is the failure? Well not the stainless steel tubes. Take a close look just below the center tube and you’ll see the rust line where the vestibule panel is rusting through. The opposite side is the collector/transfer box to the secondary heat exchanger feeder tube. Air from the blower was feeding into the collector box and disrupting the the flame.