I have configured my laptop to just blank the screen when I lock it instead of
running a screensaver (in the XFCE screensaver settings). I have also
configured it to turn down the brightness automatically after 120 seconds of
inactivity (in the XFCE power manager settings, 'On Battery'). When I lock the
laptop the screen fades to black and turns off, as expected. But after the 120
seconds have passed it turns on again, completely white, and stays on until I
unlock it. If I change the power manager configuration to never turn down the
brightness this does not occur.

I would expect it to not turn on the screen in order to reduce the brightness,
but rather keep it off when it is turned off.