Happened to me for Halo: Reach Normal playlist.
I was on 10/12 before the update and it bugged, well silly me for not finishing, I thought I'd keep it for a quick "finish a playlist" challenge in Season 6, I'd never had a playlist checkpoint bug out in the previous seasons/updates.
So I restart it, do 3 missions for a challenge, then last week resume it and do 1 more mission for Playlist Progression weekly challenge, and now this week I try to continue it again for Playlist Progression weekly challenge and it loads all the way along and then kicks me back to the playlist menu.
Not seen anything about playlist checkpoints bugging out without an update, except maybe the earlier days of MCC.
Update: Signing in a Guest and starting the mission resets it to the start of that mission, but does load in. You can then save and quit, sign out the Guest and load in Solo.
A lot better than having to restart the playlist... again.
Also, I did think of one thing that had changed since I last played the playlist, I'd fiddled with the new POV setting, that could have upset it, not that you'd expect it would.