Ah-HAH!!! It looks like the culprit is Light Flow Lite. I've been using it forever, but maybe a recent update broke something. I've sent a message to the developer to see if they are aware of the problem and/or have a fix. I'll post again whenever I hear back from them.
I have uninstalled and reinstalled the app, and all seemed to be working correctly this time. I received a contact from one of the developers (I think), and he said this:
"Could you try go into lightflow, settings and make sure non of the "additional leds found" are checked. One may be your screen backlight and it may be getting switched off by the app when you switch your screen on."
I don't recall whether or not these settings were checked previously, but if the mysterious dimming starts up again this will be the first thing I check.
It appears I'm solid now, but if anyone else has other issues, I'd encourage you sending an email to the app developer.