Just a guess,maybe the pressure[small amount]the rounds under the bolt.Big maybe.

Thats a new one to me. I'd remove the stock and check that the sheetmetal magazine well is sitting properly in the groove of the receiver, then check the stock to see if there is any flashing in the mag well area left over from molding the stock, then check that the aluminum bedding block is still firmly molded into the stock. Torque the stock bolts to 65 inch-lbs when you reassemble the rifle, and see if the problem went away.

I've heard of zeros changing with different mags in an AR.  Are the locking lugs bearing weird?  Bolt pressure??? Just random stabing. HTH