Ok I dont know how to reproduce it ingame but this might make winows force the bug to happen
Get a laptop, run arma reforger and go ingame, then go to another process and do something there for a minute, alt tab back into a game.
Sometimes windows thinks the game is a background process. Then alt+enter and it might fix it.
Now I figured that out once or twice, and can sorta make the bug happen, but the alt + enter thing somwtimes fixes it and most of the time doesnt.
Will keep posting as I figure it out so hopefully this gets resolved.