Getting credit for unplayed missions

Upon completing the fifth mission in Halo CE’s campaign, “Assault on the Control Room,” the next mission titled “343 Guilty Spark” loaded and after the cutscene I saved and quit. Fast forward to the end of the day and I go to my campaign progress. It says I completed “343 Guilty Spark,” and when I load up campaign it takes me to some random checkpoint in the following mission, “The Library.”

The only thing I could think of was between those two events I did play co-op Heroic on the second to last CE mission. I don’t see how that would essentially give me credit for a campaign mission I never played though.

Has this happened to any of you guys? I tried to be as detailed as possible to assist 343 in fixing an issue if there is one.

bump

This happened to me too, but for me, I had saved/quit on The Library and when I came back, it started me on Two Betrayals.

same

So I played through 3 missions today on ce legendary and got the acheivements for them but it says I haven’t completed them I have 5/10 completed when I should have 7/10