I have found many threads on this topic but they all seem to be closed without offering a real solution (or not one I can understand and use anyway) so I made this new topic. Like many other FP4 users I have a problem with the brightness of my screen. It randomly turns unusable dark. The brightness setting is 100% the extra dim is not on, nor is the adaptable brightness activated. battery saving mode is also not on. I am fully updated to android 13. Is there anything I can do to get my phone usable again? and yes I have turned it off and on again that works for like two seconds before going dark again 
Your problem might be slightly different, but on my FP4 I had managed to alleviate (not fix!) the issue by using the (broken) auto-learn feature of the adaptable brightness.
I repeatedly, obsessively changed the brightness level the adaptable brightness was setting (over a period…), until it eventually adopted my own, brighter settings. It took a while but it worked.
For a while it ceased setting my screen to pitch black when the room was dark(-ish), but unfortunately this only lasted a couple months, almost a year, after which it lost its training and went back to its default settings. Now I’m pondering if it is worth my time to re-train it anew. I’ll probably do it in winter when the days get shorter and there is less light.