I was doing the master chief saga playlist but when I clicked to resume on The Armory, (2nd mission in H2) it threw me on Halo (2nd mission of H:CE). Id rather not do everything again so hopefully someone will get back to me 
Had a similar thing happen to me last night, when I resumed play I was on truth and reconciliation and powered through all the way to the maw, then backed out to invite a friend and continued playing, we got to outskirts on halo 2 when it dropped him from my game, so I saved and quit to bring him back. When I tried to resume again it said outskirts, when it started to load it reverted back to halo (ce). Progress showed I have 13/47 or whatever, wasn’t too happy about it. Seriously nearly a year since this thing been launched and it still not functioning properly.