OK, well it is still listed in Improving the software of FP3
Funny, I have a theory about the culprit from my PoV. I need glasses (tried lenses) 24/7. However, they are also darkened against sun sensitivity (they do start to work better once I’m outside but they work 24/7). I wear them all the time, even when I use my FP3. So my perception of the FP3’s light (as well as other devices) is also 24/7 different.
Even then, I think the reason why I started with backlight full on because I don’t want to see myself in reflection because I find that annoying/distracting. The darker the screen, the easier it is.