What I did is I replayed the DLC, fulfilled the requirements again and it (thankfully) unlocked. The only thing that might work is if you just wait for the achievements to unlock.
This is all the Games for Gold version of FH1 on the XBox One.
Update: It sounds like new Achievements can be added on the fly - the whole system is less rigid.
From Xbox.com: "Xbox One's all new achievements system has richer detail and spans across your games and experiences, which means achievements are no longer confined to a single game.
While I was playing an app, I had unlocked a couple of achievements for it.
The last achievement that did this to me was locked for two days.