@PurpleCat It's the LCD top backlight.
Update - i called nps germany today. They suggest, that the switch for the on/off/backlit feature is somehow broken. Because i told them - with CSM 17 - 1 the LCD panel is backlit all the time - any keypress would just for the moment of touch disable it, then light all the way. The two button reset - i did it 5-times, no change of that issue.
My other two F100's are different. Its off, then any button activates it. Also, when i hit the switch to the blacklit symbol - backlight goes on.
So perhaps there's just a bit via the internal F100 firmare being inverted, i guess? Or some connector, capacitor issue might be...nikon told me,
that they don't have any active service tech guys for the F100 anymore...of course, these ppl are retired now...the F100 came out into 1999 -
i remember that time very well...
So, i just use this F100 as it is, as my daily driver F100, when i'd need the F100. Even it does look like mint, 95% new..no scratches, dings or dents, all features besides the backlit, works as it should be, flawlessly. Hopefully for a long time to come...honestly, this F100 doesn't see more than perhaps <2-5 rolls per year...sometimes a roll (b/w) inside for ages - until it's finished, yes, and then i always pull out the batteries, for storage. This F100 wasn't used the last 2 years...so the milage on this shutter is very low.