Having tested it after randomly happening upon the issue I've found it is caused by every piece of clothing (to some degree or another) other than underwear or the default clothing when you're not wearing anything in the inventory slot. Vests and backpacks also add to the clipping, but helmets for sure don't interact with the scope.

Best guess: the Zafir's camera position (that you change to when aiming down the sights) for the RCO and ARCO is put too far back on the weapon and is clipping into the shoulder, especially when moving to the right which causes the biggest clipping issue of all directional movements. I assume this has to do with some design decision to keep the Zafir's iron sights still visible in any optic with dual purposes built in. Short range holo optics have no issue and long range sights like the Thermal sight for machine guns do not have issues with this either.

Testing on the MRCO yielded nothing for me, oddly enough, but the image definitely suggests there's something to check with it.