Possibly the wrong version being completed?? You may already be aware, but you can complete the mission of legend via the weekly campaign mission, or, you can select to re-run any of the campaign missions, and legendary is an option to choose. I think (can't recall the exact details) there's a triumph for both, so you'll have at least one thing checked off, but you should just need to do the other one perhaps?? If not, a new bug perhaps... may be worth googling!