So I thought it might have been a problem of Asus, moreover it is notorious for things like upside-down web cameras and such...
On this forum people have reported similar problems, but there is no universal workaround for them:
https://forums.mageia.org/en/viewtopic.php?f=23&t=2665&hilit=screen+brightness
https://forums.mageia.org/en/viewtopic.php?f=8&t=2651&hilit=screen+brightness
However, I occasionally bumped into the solution, reported for various distros (Ubuntu and Arch) and several hardware vendors (Samsung for example). The simple thing that solved my problem completely was adding the following kernel parameter at boot:
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acpi_backlight=vendor
I am not completely sure why doing this manually is necessary in certain cases, because many people do not encounter this problem with their laptops, so if there are any experts around, you are welcome to comment and correct.
AFAIK, there is a kernel module called “asus-laptop” (or something like that), which is responsible for Asus-specific key bindings and features implementation in Linux. My guess is that it was not called properly for some reason, so that screen brightness was regulated by some generic module, which sometimes had partial functionality on a particular laptop and sometimes did not at all.
UPD.
More information can be found here:
https://wiki.archlinux.org/index.php/Backlight:
acpi_backlight=vendor will prefer vendor specific driver (e.g. thinkpad_acpi, sony_acpi, etc.) instead of the ACPI video.ko driver
(I am just curious - why isn't this option used by default?)
The recommendation is to add following kernel parameters in bootloader:
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acpi_osi=Linux acpi_backlight=vendor
or
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acpi_osi=Linux acpi_backlight=legacy