I had a similar experience when I took a flight very shortly after reaching MP Silver status and then again 2 days after reaching MP Gold. It seems that the UA kioks at the airport take some time to synch up to the correct status.
It was a simple matter of asking one of the UA personnel in the area to do some jugglery on their computer and they printed out the bag tags for free. They did check the baggage allowance on the UA website using my MP number first, before agreeing to get it done.