I started the map Fort Brachsenbrügge with the counter showing 29 of 30 art collected. During the run I picked up 2 art. But after I finished the map it still showed the 29 of 30. Even stranger was, that the counter for the total art collected increased correctly by 2 (from 435 to 437 or something like this). I guess that also happend with a few other maps because there are some more who are sitting at 29 of 30 (but I couldn´t play them right now and don´t know if they are truly at 29 or also bugged). I gonna try this as soon as my time permits.
I also looked around a bit but didn´t find any other topic with the same bug, so I guess its a new one?
Also: I played the mentioned map with a friend who just started the game on a low difficulty setting (hard), I don´t know if this is related or not.