As expected. As soon as “Mission Complete!” is displayed, the mission is considered over and the result is recorded. It’s the same reason why you can press “Skip >>” and still see the mission results.
Actually, the server won’t allow you to do that.
Regardless, bug is confirmed. This happened because after being disconnected, you re-connected too quickly to another server (which hadn’t received your result yet). This will be tough to fix – on the list.