I ended up here because I too had about 8 acheivements that for whatever reason did not register and after trying a few of the suggestions, this is what worked for me.
1. Save whatever you're doing. (I don't believe it needs to be a new save file.)
2. Don't just shut off your console using your controller and turning it back on. You must reboot your console which means holding your finger over the power "button" on your console for 10 (maybe 15? I can't recall exactly how many are required) seconds. Once it completely powers down, turn it back on. You'll know if you get the green XBOX start up screen instead of straight to the dashboard.
3. Return to the game.
Almost immediately, the acheivements started registering one after the other once I did this. I suspect when you don't do this from time to time, and I am not technically savvy
whatsoever so don't quote me on this, that it's losing the connection somewhere to EA's live service (which explains why Microsoft kept saying it was on EA's end to the people who contacted them). I noticed that it *had* been giving me a message that was telling me that I needed to log in but I had been kind of ignoring it because I was already in my Xbox account and I couldn't figure out what I wasn't connected to since I was clearly online. I suspect that was a clue to why it wasn't registering the acheivements.
Anyhow, I hope this works for those of you who stumble upon this post.
