I was probably the first person this ever happened to, I was in talks with Ruger Engineers/Head of their PCC product line guys. This is a design flaw with the takedown pin. They use a really tapered pin to seat into the receiver, unfortunately it provides no ability to lock it into place. I find that the "locking" nut actually does not prevent this at all. It's easily replicated by pulling the fore end into your shoulder and twisting at the same time. I checked out a newer PCCarbine at the gun shop and it seems they may have a newer style takedown pin that should fix this. I ended making my own takedown pins that had no taper and milled the receiver to match. I went from barely getting over 5 rounds before it would break free to 12,000 with no issues. I hope they get this fixed for you (Mine was back and forth for two months with Ruger before they gave up and offered me a refund etc, I ended up fixing it myself). Video back from April 2018. https://www.youtube.com/watch?v=cST_3Ds7Dnk
I no longer use the Ruger in competition since mag changes were holding me back in classifications. I now run a JP with zero problems 5K+ since mid December.