This is the part that doesn't make sense. A pan coated in oil or fat, baked on and polymerized (or not) shouldn't be flash rusting. Something else was afoot.
That was my point. Apparently not enough oil adhered to the cooking surface, and the water vapor coming off the gas burner caused the cooking surface to rust.
I deliberately did it the exact same way after stripping, cleaning as I described, and letting it go another initial round of seasoning (400F for 1.5 hours, upside down in oven, let cool in oven, reheated oven with pan still inside) I did not experience that problem again.