During the entire evening when playing I ended up getting the Mid-Air badge for every single kill I did, even if both me and the victim were standing on the ground. It started happening after I indeed got a kill while being mid-air. But after that it seemed to trigger for EVERY kill. However, this only happened on the map Underground. Every other map was fine.
If it has any meaning, I was playing Vassili when I got the initial kill and following kills.