My question is, what determines which way is “up” when blender aligns something with a “halo” face orientation to the camera.

I’m currently having a problem with when I create a object with a halo face orientation during run time, it aligns different than when I run the game with it already in place.

You would have to select your object, go in edit mode and orient the plane towards local X-axis.

yeah its a bit messy, in edit mode, rotate it 90 on x, then rotate it on z until it does what it needs to do (its either -90 or +90)
just keep it rotating until it works.