I use a 4k monitor but run the game at a 1440p resolution for speed reasons (Antialiasing).

When starting the game (in fullscreen mode), the game opens in a borderless window in the center of the screen half the time, the other times, it opens in fullscreen but the mouse is confined to a rectangle that is not the full screen.

The monitor is set to 150 % scaling (in Windows settings, Display -> "Scale and layout")

Turning this option off makes the problem disappear.