Comment # 7 on bug 1180591 from Takashi Iwai

Thanks.  Judging from the given information, it's nouveau and the backlight is
controlled solely via ACPI video driver.  It's a bit surprising that the
control doesn't take any effect in ACPI while it's exposed...

Maybe narrowing the regression range might help.  There are many kernel
packages available in my OBS repo, e.g. home:tiwai:kernel:5.4,
home:tiwai:kernel:5.5, etc.
Could you try installing those old kernels one by one, and check which kernel
started showing the bad behavior?

Before testing that, it would be safer to increase the number of installable
kernels in /etc/zypp/zypp.conf.  Edit the file and add more entries in the line
defining "multiversion.kernels".