I have 2 monitors, one 4k downscaled on the left and one 1080p on the right.

The desktop on the left is apparently running at 1px less then it should be causing issues like

Are you using X11 or Wayland?

When you consider that 'consumer grade' 4k is slightly less than 4k resolution this could be perfectly normal scaling. X11 cannot run per monitor scaling as the desktop under X11 is rendered as one SLS.