It actually starts, and then turns off. I didn’t notice it before you drew my attention.
That does sound like DPMS (“vesa display power management signaling”) shenanigans though.
Maybe you can disable XFCE’s display power management stuff completely? Systemd’s logind (/etc/systemd/logind.conf) can do (and does by default I think) suspend on lid-close without any window manager involvement at all, works fine with i3 here. So disabling XFCE’s stuff probably “only” messes with your monitor not going standby after a while, and you can maybe use xset or xscreensaver and set this by hand (after making sure it’s actually properly disabled in XFCE, so XFCE doesn’t override that stuff).
Found this about how to stop xfce4-power-manager and disable DPMS: