Slitherine

I just finished ALLIED Corps, with only British core units. I never bought an American unit, never upgraded any American non-core units either. The core is 100% British, always has been, as the save is marked as such to remind me. The core end game is the same core as I had at the start, filled with Brits!

However, on completing AC, no achievement popped. I did the CTRL-Shift A thing and it confirms that no cheats were used, no american units were bought (and weirdly, no british). Thinking I needed to trigger that value to 'true', I disbanded a unit on the last turn and bought a british unit on the map to switch it to 'true'. Still nothing.

Any ideas? I slogged hard for that achievement, like the sad sap I am! So I would like to get it without having to slog through the entire game again.

... and I have exactly the same issue.
Finished the whole Allied Corps Campaign, Field Marshall difficulty, and without buying any US unit: no achievement.

The achievement says "Win any Allied Corps campaign without buying any American core units. Difficulty Colonel or higher"
I tried then to replay only 1 campaign in the Allied Corps, Colonel, the D-Day 44 campaign, taking great care of purchasing nothing: still no achievement.

After 484 hours in the game, according to Steam, and trying to get all achievements but the multiplayer ones, I really wonder what I did wrong with this British (and I suspect US Victory) achievements...
Here is what ctrl+alt+shift+a tells me at the last game of the campaign:
https://s6.postimg.cc/dtmgs16cx/image.png