The 4x4 match against the computer ended in defeat, although the opponent had only 25 points left by that moment. A "sync error" was displayed, but the trophy points were successfully awarded, and the connection with the rest of the players was not broken either.
After that, not at all, but at many achievements, the filled counter was reset. For example, for the "German Battle Medallion" or "War Games III".
On the Steam forums, people in a thread I created are talking about having the same problem. Will this issue be fixed soon?