A workaround for the people who like OBS and want to record/stream MOHAA in fullscreen without problems and without the need of record the whole desktop.
(See Appelpitje's post and Slimbips's post)
While in game and after loading the map, type "exec obs" in the console (needs to be executed every time your game loads a new map)
Done.
How it works?
Basically, it places a permanent invisible fullscreen menu in the screen, so OBS can "see" MOHAA properly. (For some reason OBS only detects mohaa when a menu is open)