Unless I am mistaken, the flight should be in progress. However, when trying to join multiplayer, I can see my user load, be set as fire team leader, and then an infinitely playing loading animation ( UNSC icon ), which never seems to resolve, and I do not see multiplayer UI, only season pass UI element / fire team UI element, and the bar at the bottom.
This behavior is different from my past attempts to access the MP portion when flights are not in progress, in which case the pvp ui would load ( game type, matchmake, join, image , etc ) but would fail to start matchmaking.
UPDATE AND SOLUTION;
I uninstalled / reinstalled in case the issue was due to the build version ( currently on 1.3167.15457.0 ).
Reinstalling fixed the issue, however:
I launched while the install was in progress, and also saw the same failure to load.
I am guessing this has to do with may pvp assets missing / not being installed yet, and some missing UI code to handle that case.