Same issue here since the upgrade to 4.20, monitors go blank after 10 minutes.
I don’t have xfce-screensaver installed.
‘Put to sleep after’ in Power Manager for AC is fully disabled.
xfconf-query -lvc xfce4-power-manager
/xfce4-power-manager/blank-on-ac 0
/xfce4-power-manager/blank-on-battery 0
/xfce4-power-manager/brightness-on-ac 9
/xfce4-power-manager/brightness-on-battery 9
/xfce4-power-manager/brightness-switch 0
/xfce4-power-manager/brightness-switch-restore-on-exit 1
/xfce4-power-manager/critical-power-action 3
/xfce4-power-manager/dpms-enabled false
/xfce4-power-manager/dpms-on-ac-off 20
/xfce4-power-manager/dpms-on-ac-sleep 0
/xfce4-power-manager/dpms-on-battery-off 5
/xfce4-power-manager/dpms-on-battery-sleep 0
/xfce4-power-manager/general-notification true
/xfce4-power-manager/inactivity-on-ac 75
/xfce4-power-manager/inactivity-on-battery 15
/xfce4-power-manager/inactivity-sleep-mode-on-battery 1
/xfce4-power-manager/lid-action-on-ac 1
/xfce4-power-manager/lid-action-on-battery 1
/xfce4-power-manager/logind-handle-lid-switch true
/xfce4-power-manager/power-button-action 0
/xfce4-power-manager/show-panel-label 1
/xfce4-power-manager/show-presentation-indicator false
/xfce4-power-manager/show-tray-icon false