I recently completed the Halo 2 Heroic Campaign Playlist with a friend of mine. For some reason when you play the playlist in co-op, it skips right to Cairo Station, which makes the finalized count when you complete the playlist 13/15 missions completed. After finishing up, I went back and watched the first two missions, giving myself the 15/15 completion on the playlist itself and the overall campaign. The Halo 2 playlist counter still shows zero, and the MCC thinks I haven’t completed the playlist, so I haven’t received the achievement for it.
Aside from simply replaying the playlist solo, is there anything I can do to try and fix this?