The front of the pylon is the plane of the end zone.
If the the ball is fumbled forward into the pylon (even from an angle), it's not possible that the ball was in the end zone before hitting the pylon.
If you can draw up a scenario where the ball was fumbled backward from the end zone into the front of the pylon, you will win the Nobel Prize for physics.
You are absolutely correct from a physics standpoint. However, the rule is that if the ball hits the pylon, any side of the pylon, it's considered to be in the endzone or have gone through the endzone out of bounds - physics be darned. Perhaps an official on the board can cite the exact rule.