I have the same issue, also on Xbox. All three of the new 'Secret Achievements' show that I have completed them 100%, yet haven't unlocked for me. Though before I played through the new campaign, I had checked to see if there were new achievements, as I like to complete them, and there weren't any there. It wasn't until today that I saw that these achievements exist.
It appears to be linked to Xbox as the achievements were not live when the DLC went live. So the Hub shows 100% but don't pop.
I think you are right. I finished the first 2 and did not get the achievements for them, but I finished the last one tonight and did get it. I replayed the first mission, but still did not get the achievement for it.
Same for me. Played the first Mission on december 13. Showing 100% but is not popping up or counting. Played the second Mission yesterday and it popped right after completing it.
