SOLVED: Playback issue

[danishduckling]

I have an ultrawide monitor with a 3440x1440 resolution, and when I play any content it shows it like this photo with black bars all around it, even when it's a 4K video like the above example - why is it not scaling to the window when there's plenty of room?
I've tried the different options in the menu to stretch the video, but none seem to be very good, either the image looks weird, or it doesn't fit the window correctly - and with all of them, it seems to stop any HDR and just show SDR instead, except for when I allow it to play like in the photo above.