To close the loop on this, I sent the board back, FM declared nothing was wrong with it, and shipped it back to me without any logical explanation as to what was happening in my videos. I was not happy to say the least. After throwing a bit of a fit, the team escalated to engineering and I finally got an answer.
If you start the board with your front foot at a diagonal, when you put your weight on your front foot, your heel comes up off the back side of the sensor for a split second, which causes the board not to start. The fix is to put your foot more perpendicular to the edge, so when you lean forward the side of your foot pushes down on both sides of the sensor panel.
I've requested that engineering put in a grace period on the front sensor so that if the sensor had weight on it in the last half second or so, that it still considers you to be on the board and that it starts up properly. Let's hope they commit to fixing it, as many of us probably have had this issue happen to them at least once and wondered why the board didn't start.
I also made a video explaining all of this: https://youtu.be/rnMut5_p4Bc