Cannot change resolution in windowed mode

[6./ZG26_Emil]

I have a 2560x1600 monitor but play the game in 2560x1440 so I can record gameplay for youtube. I've been away from the game for a while and just updated it but when I select the 1440p resolution it is still fullscreen, before I had the black bars top and bottom showing that it was in 1440p mode.

 

Does anyone have any ideas how to fix this?