The lobes on a flat tappet cam are angled front to rear and rear to front in order to spin the lifters in their bores. This is not visible with the naked eye but it is there. When a FT lifter fails to rotate for various reasons is when the cam wipes a lobe. A roller cam has a perfectly flat lobe surface as a roller lifter does not rotate it is stationary because of the roller.
Mixing and matching these components makes for a quick disasterous finish.