So if you set the dimmer to medium like so...
...then leave the scanner alone, eventually this setting will get changed to high? Does your scanner have the latest firmware?
It is not inconceivable that the problem could be caused by the SD card or firmware. I would try reformatting or replacing the SD card and reloading the firmware.
That makes sense. When you turn off the scanner, it saves the last brightness setting of the display. When you turn the scanner on, the scanner brightness should be the same as when you turned it off. If you still have the problem with the brightness changing, try my previous suggestions.