According to the 'User Guide':
"When you first turn on your computer, it may take several minutes for the
screen brightness to “warm up,” especially if you’ve chosen a high setting."
Note that ambient temperature changes will also affect these older LCDs. In that respect, it's normal.
Also, those old FSTN displays were pretty terrible. That aside, I wonder if your PRAM/'Backup Battery' is flat or failed.
To test that, set the date & time, then shutdown and unplug the mains power AND any user-removable batteries for about 10 minutes. Then plug in the mains power and power on. If the date has reset to default (1972 ?) then you need to leave it connected to the mains power for 48hrs and re-test. If it fails again, you need a new internal backup battery (or you can just live with it).
It's possible that the LCD display or the interconnect board or inverter board (all located within the display housing) has a fault.
Finally, if you haven't already, do yourself a favor and google "powerbook_500_series.pdf"
