Its not prop position that's making the engine run backward. Its not spinning the prop forward fast enough that first revolution for it to carry past TDC as the fuel ignites, and then, the engine reverses.

That's one reson I use the reverse flip. I'm purposefully using the effect, to have the "backfire" flip the prop forward for me.

Prop position can effect how effectively you can hand flip the engine... and the most efficient forward flip comes from the prop being parallel to the ground (3:00/9:00) to just a bit past (2:00/8:00) as the engine enters compression. If you tend to have your hand flips go backward, move more toward 3:00/9:00.