On 21:9 display phones, such as Sony Xperia 5ii, there is a white bar across the bottom of the app that ruins the look a bit. Is there a chance this could be fixed?
This is outside the influence of Plexamp.
This white bar is painted by the Android OS. If you activate the Dark Design of Android, it will be a black bar instead.
The only other option would be to replace this thin bar with a thicker one, which has the traditional 3 navigation icons for Android. (The switch is somewhere in the Settings app under Accessibility.)
I thought as much. The annoying thing is that I am on the dark mode, I wouldn’t mind if it was black, like most other apps - it just stands out a bit.
Try disabling the dark mode. I have it disabled and the bar is black. 
