Monitor Resolution Is Not Filling The Display

OS: Trisquel 10 Nabia
DE: Mate

I recently got a 3rd monitor for my home workstation. The other two monitors both have 1920x1200 display resolution. My new monitor has a display resolution of 1920x1080.

I was able to use xrandr to set all of the displays to their appropriate resolutions, but the 3rd display; despite being set to 1920x1080 does not fill the whole display. THis issue seems to be specific to Mate (Trisquel's default), as I am able to see the full display when I run Gnome.

I have already tried in xrandr:

xrandr --output DP-1 --set "scaling mode" "Full aspect"

... But this just keeps the aspect ration and puts a black border all around the display.

I have attached an image of what I am describing.

I like Mate and would like to keep using it. Do you all happen to know how I can make my 3rd monitor fill the display?