Solved. Installed Mid Industries QD.

I have a Sig MCX that I got from a third party and the Magpul QD mount in my slingster just wont stay locked in either side. You can rotate it or pull it and it'll come off. I spoke with Sig about it and they say that its not something warranty since I did not get them from them. I also saw a product made by Poi-Tac ST-MCX which includes a hook with the QD mount but I am unsure if due my issues it will stay locked.

I dont want to replace the whole lower for something so minuscule as this. Solutions?

Did you try a different sling.  Mine stays fine but I can't use it because the surrounding material is the same height as the stud hole so the sling wears all the finish off in a semi circle around the hole and I'm not willing to allow that.  Poor design.  If theyd just extended the hole out 1/16" the QD holes would be usable.

I'll try and see another sling. If not, I think I'll be forced to buy another brace that has its own attaching system.

I had this exact issue. It was the QD Mount or maybe both being slightly out of spec but it didn't allow the ball to go deep enough. Changing to a different one solved the issue.

Which one did you changed to? I am using Magpul QD mounts on both ends.