Android app showing black borders overlapping the

问题:

I've encountered a strange problem with an android app designed for phones when run on a tablet. The UI is scaled correctly to the device, but there's this big black border overlapping the edges, as if it's trying to show some sort of phone-sized version of the screen.

Oddly, when using Android Studio's Layout Inspector to try and see what the black border is, it only shows the UI we want to have, with no sign of the black border:

I've fiddled with a bunch of settings and tried googling for the issue in every way I can imagine, but can't find anyone with a solution to the issue. Has anyone encountered this or does anyone have an idea of how to fix it?