This just worked for me.
Makes sense. I have a new monitor and have been playing around with the video settings frequently.
Solution:
From Arma 3 Launcher > Launch Arma 3 [Without Mods] > From main menu press [Left Alt + Enter] to go windowed > Open Server Browser > Launch Server > Select Role & Join > From in game press [Left Alt + Enter] to go fullscreen
Will update if this method fails.