This commit https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0cafc82fae41531b0162150f9a97f2c74f97118f breaks KDE and any other software that assumes setting 0 will induce the minimum brightness. Maybe it would make more sense to set the brightness level to "1" if userspace requests brightness level "0" so programs still function correctly.
If desired the call could still return false to indicate it was out of bounds, but still lower brightness.
In addition, even though there is a write error, when you read back the brightness value, it shows that the backlight is at level 0, while its brightness has actually been unchanged from before:
echo 0 > /sys/devices/pci0000:00/0000:00:08.1/0000:05:00.0/backlight/amdgpu_bl0/brightness
bash: echo: write error: Invalid argument
This should fix it: https://patchwork.freedesktop.org/patch/260537/
Closing this since it has been fixed.