Brightness changing even when auto brightness is disabled

I have a new (July 2021) Dell XPS Laptop running Ubuntu 20.04.3 LTS I have turned OFF Automatic Brightness and manually set the brightness that I want. But after rebooting often the brightness resets to near maximum brightness, and I have to manually change it back to the lower setting that I prefer. How do I get it to stop doing this?

One solution may be to set up a quick little script that will save the brightness value when the system shuts down, then configure the system to reload the value after booting. It's not perfect, but will do the job in a pinch.

Here's how you can set it up:

0. Open Terminal (if it's not already open)
1. Create a file in /etc/rc6.d, which contains actions to be performed when shutting down. For the sake of this example, the file will be called K99-save-brightness:

   sudo {editor of choice} /etc/rc6.d/K99-save-brightness 
   

   ^{Note: Be sure to replace {editor of choice} with your editor of choice.}
2. Copy the following into the file:

   #!/bin/bash
   cat /sys/class/backlight/nvidia_0/actual_brightness > /etc/acpi/backlight-brightness
   

   ^{Note: Be sure to replace nvidia_0 with the appropriate name that is found in /sys/class/backlight. It may be intel_backlight, nvidia_0, or something else entirely.}
3. Save the file and ensure it is set to be executable:

   sudo chmod +x /etc/rc6.d/K99-save-brightness 
   

Now every time your system shuts down, the value in actual-brightness will be written to the /etc/acpi/backlight-brightness file.

Next, you will need to run a little script after your system finishes booting. This can be accomplished via crontab like so:

0. Open Terminal (if it's not already open)
1. Edit crontab as the super user:

   sudo crontab -e 
   

2. Add this one-liner:

   @reboot (sleep 10 ; cat /etc/acpi/backlight-brightness > /sys/class/backlight/nvidia_0/brightness
   

   ^{Note: The sleep value can be anything from 2. When the value is less than 2, there is a higher probability that the backlight setting will be overridden and reset to maximum. Of course, be sure to replace nvidia_0 with the actual backlight device that you will be controlling.}
3. Save the crontab
4. Reboot to test

This is certainly not an ideal solution, but it's one that does work on some of the notebooks that I help support at the day job. This should work on every modern version of Ubuntu from 18.04 to current.