Hmm. Well, a reflash wasn't needed, but I'm not sure what made it start working again. I posted almost an hour ago and it is normal now.

Prior to posting, I rebooted and (un)plugged the PSU a number of times, to no avail. I did more of the same after posting, and prior to the final reboot, adjusted the Brightness Period and Switch Off Display timer away from and then back to my original settings.

The only other thing to note is that due to the PSU being plugged in some of this time, the battery is in a higher state of charge than when the problem manifested itself.

I also noticed that with the device in the quirky dimmed state, there was a weird flickering of the backlight.

I wonder if anyone else has experienced such a thing?