On my previous system I played this game in 1080p on a 1080p screen and it ran fine in fullscreen.
Now on my current system I have a rx 570 and a 4k screen, it runs fine in 4k but at a low framerate so I wanted to change it to 1080p or even 720p but when I do that I get some sort of window that only fills the upper left corner of the screen.
I've tried a few things like running in windowed mode or other resolutions but the problem is still there, I only get fullscreen in 4k. :/

I thought you could be on the right track since I turned it on, but with it turned off it still didn't work.
But I solved it, apparently MSI Afterburner was causing this because with it turned off I have the game fullscreen on display in any resolution.

Thanks!