I use this set up exclusively. Problem only started occurring after Marksmen DLC update. I just recently purchased the DLC pack in partial hopes the problem would be solved. Still exists 100% of the time. I have taken off all gear and tried each piece at a time, first thinking it could be the tip of a rocket launcher on the back. It is definitely the backpack you are seeing through the scope.
Even oocurs when prone on runways, definitely NOT grass but backpack.