Mine had similar issues two days ago, when shooting in zero-degree temperature, with a couple of Pmags Gen 2. The fired round woud eject, but a new round would not chamber. It became a bolt-action gun. From time to time the fire round would not eject, and a new round would try to climb up the feed ramp, then the gun and mag got stuck. This had not happened when it was warm.
However the problem went away after I borrowed a metal 10-round magazine and tried again. All rounds shot, ejected and fed perfectly.
I will try again in a couple of days, and will bring different mags.