KDE Plasma: Maximized window on second monitor leaves large gap
I had the same issue and was able to fix it by changing and re-changing the display alignments in the display settings.
So, it was lined up, I moved it around, pressed apply, then moved it back to original position, pressed apply. It's the same settings I started with, but it got rid of that gap.
I should note that this was on a different version than yours (5.16.9, Fedora 31).