Campaign not saving or resuming on current mission

I had the same problem after downloading the lastest update. Instead of resuming the campaign, just restart the current mission. You'll lose the current checkpoint but you'll be able to continue the campaign.

Doesn't save, kicks you out mid-mission so you lose progress, resumes the mission in the middle but doesn't retain your score. Single player campaign is a steamy pile.

Correct, you can save and quit anytime solo without finishing the mission, but you must resume campaign. You can't save and quit, then load another level, then go back to a check point on the level you haven't finished on Legendary.

If you do the Save/Quit menu option in your mission, when you return to the game later you should select
Resume from that Halo's campaign menu.

If you just go to Missions and pick that level out again, it will load that mission from scratch.

Classy ; )

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Go to the Campaign Lobby and Select Mission ---> The mission you were up to ---> Mission start ---> finish it and carry on.

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Once you stop playing this time, press Start and Save and Quit, then go Resume Campaign instead of anything else.

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If you are actually having an issue instead of having made a mistake, then I apologise but you'll figure out whether that's the case when you next try to load your progress.