It looks like FN had more of an issue with an aftermarket muzzle device being used which voided the warranty.
In regards to suppressed use vs. adverse use, it's pretty easy to see what the different gas settings on the gas block are for if you take out the adjustable part. Compare the port holes. The 12 o'clock Normal port is much bigger than the 10 o'clock port. Also while in the 10 o'clock position the two small ports on the side line up with a channel in the gas block. All of this should lead one to believe it's for suppressed use to help choke the gasses from back pressure and bleed pressure from the piston chamber. If it was for adverse conditions the 10 o'clock port would be bigger for more gas pressure to help cycle the action under adverse conditions and the small ports wouldn't exist. Whether it's heresay or an actual FN rep said it's for adverse, it's wrong.