Comment # 14 on bug 1143755 from Takashi Iwai

(In reply to Peter Luladjiev from comment #12)
> (In reply to Peter Luladjiev from comment #10)
> > "Does the backlight control itself work like before?" - Sorry I don't know
> > what that means. 
> > 
> > However I've added systemd.restore_state=0 kernel parameter and now it works.
> > 
> > But do I need to leave it like this?
> 
> Okay never mind, went black again 20 minutes after a boot.

It implies that the desktop environment lowers the brightness for power-saving.
I don't know what change in kernel makes the difference, though.

Check the values in /sys/class/backlight/* (both brightness and max_brightness)
at the dark state on the 5.2.x kernel.  Then boot with 5.1.x, and check the
values there again.  If the max_brightness differs, it might a cause of
inconsistency, for example.