Was the oxygen mask, on this flight, out of order?
According to this Airbus training manual, pages 10 and 11, that's the test door stop.

As you can see in the video you added, resetting the masks is a pain, so the little red stop is there to catch the door and stop it from fully opening.


  Manually operated doors stops are fitted on the container doors to allow an operational test of the mask release system.
  When set to test position the door stops will prevent doors from fully opening and masks from dropping out of the Container.