More than likely not seated in correctly. The top pin doesn't have quite the positive feel as the bottom pin when it clicks in. I can see how it can me easily missed.
Its a Gen4 17. He just recently got it back from Glock this past week for warranty work so I doubt that any of what you mentioned is the cause.Maybe an indication the pin has been removed incorrectly over the life of the frame. Check the locking block for cracks too. Might want to check the trigger pin for breakage too.