Fullscreen mode in PIE 2nd monitor Game Viewport settings

Hi i want to play my game in the editor on a second monitor.
Under Modes /Advanced Settings there are settings for new Viewport resolution and New window position. Setting these only works once. The next time they are set to seemingly random values. Why? I understand that if either display is set to greater than 100 scale in windows this breaks it. Bit I’m not doing that.

M1 is 3840x2160
arranged below and to the left is M2
M2 is 1920x 1080.

If I set view port resolution to say 2000 x 1200, and
New Window position to left -10 and Top to say -50 i would assume that the monitor is smaller than the viewport so I would get something fullscreen is. But what happens is Unreal does whatever and the next time i look at the settings they have changed to some random value.