I’m terribly sorry for not doing this before blurting out my previous response, but after going through pages and pages of my log notes, I came across something that might be of help to you.
Using info from this forum page and @rscm’s posts:
you will see that there are four possibilities for acpi_backlight –
acpi_backlight=video
acpi_backlight=vendor
acpi_backlight=native
acpi_backlight=none
According to my notes, what worked for my Legion 5 Pro laptop was acpi_backlight=video. Perhaps of even more importance to note was that, for me, this only worked in Dynamic mode. I could not get any control of the screen brightness in Discrete mode.