Fullscreen mode is automatically toggles on in Development build

I need to package my game for testing, but it runs incredibly slow because it automatically sets everything to “epic” and makes a window much bigger than the viewport I work in.
I have a DefaultGameUserSettings.ini file in [MyProject]/Config folder with following settings:
[/Script/Engine.GameUserSettings]
bUseVSync=False
ResolutionSizeX=800
ResolutionSizeY=600
LastUserConfirmedResolutionSizeX=800
LastUserConfirmedResolutionSizeY=600
WindowPosX=-1
WindowPosY=-1
bUseDesktopResolutionForFullscreen=False
FullscreenMode=2
LastConfirmedFullscreenMode=2
PreferredFullscreenMode=0
Version=2