If your display device reports that it can handle a resolution higher than its native resolution, the game will render everything at that resolution and then scale it to the native one, resulting in distortion of some graphics (e.g. the HUD map is not circular, characters are tall or fat vs what they should be) and borders that should not be there on others (e.g. intro videos). This particularly affects some 4K monitors and TVs.
e.g. Many native 3840x2160 16:9 displays report they can also handle 4096x2160 as an input. The game will see that and render to it, even when you choose 3840x2160/16:9 in its settings.
To fix this, you need to delete the 4096x2160 mode (or whatever it is in your case) so the display drivers no longer see it. The following guide explains how in simple details:
Confirmed to work with LG B7 and C8 OLED TVs, as well as ViewSonic VX2475 monitors.