Using the pre release version (1.2.9) with the latest version of GTA V.
After setting a custom first person aiming FOV in conjunction with a custom first person FOV (named "fov" under the in the .xml file>), aiming using the regular aiming mode (not iron sights) or peering out of cover in first person causes the camera to zoom way in, as if the FOV was at 15 or something similar. The game is then stuck using this FOV when you aim until you use the scope of a sniper rifle, which seems to reset it back to what it's meant to be on the .xml file. Reloading the file using F11 in game does not help.
If you are using the default max FOV of GTA V by moving the "First person on foot FOV" slider all the way to the max, this issue does not occur.
If any further detail is needed I would be happy to provide it.