That looks like an older (pre-2013 or so) CMMG stainless steel frame (no notch for the forward assist lug). In any case, what you have experienced is age related creep (or 'time warp', if you will). I have a 2010 vintage CMMG stainless steel frame that by 2017 had warped so much, the bolt didn't fit in the upper any longer. I contacted CMMG with the photo showing the creep issue and they sent me a new frame free of any charge.
Prior to contacting CMMG, I did try, without success, to bend the warped frame back into the correct position. The effort only resulted in my referencing the part as having had an incestuous relationship with its female parent.
MHO, YMMV, etc.