You will probably have more luck posting in the game forum you're referring too. It might be a bug that the devs are unaware of, or other players will have found a fix for that particular game.

Ask game devs and you most be online and have Overlay working, some games did this in the past , it is game devs that set how the trigger this, we have also hear of ppl play in offline mode and then they get online the get them.

ps.
last resort use SAM, just make sure yuo dont have a online game or reward from them, that could be seen as cheating, steam might dont care but who said game devs like that.

Many forget scoreboard at steam compare to other players, even that can be debatable as cheating, we just simple forget that scoreboard.

Same with in game scoreboard that trigger acviments, you actual did not do.
So do watchout for them greyzone, many game devs might not care about it, untill some game devs do.


ps.
And in the old days other play use Total war level achivement that you could play at Legendary level or lesser before they will play with you, So SAM can actual give backfire on yourself.

There are players inm steam you do not will get away with fake achivements, how else will you be judge that you can play at a specific game level.

i only tell you this, so you dont just use SAM, withou consequence depend on other steam player viewpoint at this. ( even is steam or game devs give a crap about it. )

This is not about me and my post , this is you and what other steam user will block you for pretent play at Legendary level then you cant. so watchout there are steam user that use achivement for proof of Merrit merdals.

and we are back at do not do it on such online games.