debian xfce screen resolution reverts randomly and after suspend

I've come over to Debian from Ubuntu - great! I'm using xfce4 as the desktop environment. One issue is my chosen screen resolution reverts to a default - both randomly while I'm on the PC (just happened now while typing this!), and it also reverts after the PC returns from suspend. I choose 1920x1080 in Settings->Display, but the system reverts to 3840x2160. However, the system saves the 1920x1080 config when I reboot or stop/start. I'm guessing there may be a setting in a file I can tweak?