I completed “In the Enemy Shadow” mission on both HARD and BRUTAL difficulties under 4 minutes and I still have this achievement locked. I’ve tried it several times with a stable internet connection (I unlocked other achievements along the way). I also completed this mission without save/load. So I don’t really know what else to try.
I did lost my internet connection the first time I actually fulfilled the conditions for this achievement, and then I restarted the game and got “Game client version mismatch with selected region” error, so could it be that its bugged out for me for good?
UPDATE
I removed corrupt save files, replayed again and it worked. So if anybody encounters a similar issue, I recommend removing save files, which you are not able to load. This could occur because of network connection issue, as in my case, or some kind of game update.