This question has been asked many times by many users, but there is no definate answer and it still remains an open bug even after all this time. This appears to be a problem with Ubuntu on some selected number of laptops. I'm using Ubuntu 20.04 for almost 2 years now, but this problem arose in last 1 or 2 months. Most solutions I have read online regarding this are to run an automated script to lower the brightness on every startup. But that is not a native solution. I would like a permanent fix to this because this bug didn't exist for me before 1-2 months ago. My other laptop which is also running 20.04 doesn't have this issue. I'm running the latest version of Ubuntu 20.04 on Dell 5559 laptop with an SSD.
The problem is that there's a process that runs at boot, and also at shutdown, that tries to restore/save backlight settings. One some configurations the restore happens incorrectly, and can cause the brightness problem that you see. Here we will add a minor kernel option so that this process doesn't restore an invalid brightness setting.
Rather than using the script that you found, (that I probably wrote), do this:
sudo -H gedit /etc/default/grub
Locate the line that looks like:
And change it to:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash systemd.restore_state=0"
Set your brightness to the desired level.