So, I checked my past games. I won on 20 maps actually and the latest one was Battle Islands V, but achievement is -for some reason- assigned to the next game. Therefore, detective achievement goes to Master Jz :) I don't think it is worth reporting though, just a minor bug.
There is actually cca 36 hours of difference. I think that Krzychu have a point - when achievement must be rewarded in ranked game, and completed during practice, it is assigned right after the next ranked win.
One thing to note is that different versions of the same map count as different maps, for the purposes of this achievement. So you may have won in an old version of the map, then win in the new version, and it counts as another map.
Another thing that could cause this is the archive database being slow. To count how many maps you've won on, WarLight needs to ask the archive database about maps you've won in archived games. However, the archive database is sometimes temperamental, so when you win a game, sometimes that query times out and it will skip awarding the achievement for that game. Then you win another game, but the archive database is working fine that time, so it counts your maps and sees you're over 20 and awards the achievement.
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