Starting at Online Achievements Interface Steam.cpp:110 it seems to treat all submissions as "unlocked" without any regard for the actual stat progress.
Is it possible that you could've lost connection to Steam at the time you acquired the Terror Gauntlet (I don't have the game so I don't know what triggers this)?
If so, it may have missed that "trigger" and it would not have registered with Steam.
If you notice that stats or achievements do not appear to be tracked, or previously attained stats and achievements are missing, that is a good sign that either stat tracking is temporarily down or that you may have an Internet connection issue.
During this time, your stats and achievements may not be recorded.
Unfortunately, that means they will not appear later when everything returns back to normal.
This is what I would assume first would be the culprit.If you set it to 10 I would think that equals 1 and therefore 100%. For quick testing, I've been using the blueprint node Write Achievement Progress.Try submitting your stat with the value being between 0 and 1 instead of 0 and 100. I tried creating a new stat that is a float, associated it with the achievement and published my changes on steam.According to the Steam docs, I shouldn't even have to write an achievement, only the stat: My first thought is that you have registered the stat in Steam with a value of 0-100.The achievement takes a percentage (see Set Float Stat above), likely meaning a value between 0 and 1.From my understanding/research this functionality is broken for Steam OS, you have to use their Leaderboards for incremental Achievements.