Opening a Fortnite session from UEFN forces it into a 1920x1080 window on 1440p screens. Doesn't allow for changing for fullscreen.

Thanks for the answer. I wanted to reduce the screen resolution of the fortnite session. It seems that Epic Games decided to separate the screen resolution settings for the game and the editor, so when the session starts, it picks up the resolution settings from the editor, and not from the game settings, which we tried to change xD