The function keys were not working properly on my Keychron K8 when in wired
mode. They would only function as their desktop-environment equivalent, even
when combining with the Fn key (e.g. F1 would only decrease brightness, F2
would only increase brightness, etc.).
Setting the hid_apple module option of fnmode to 0 worked.
/etc/modprobe.d/hid_apple.conf:
options hid_apple fnmode=0
You can also do this ephemerally by overwriting the
/sys/module/hid_apple/parameters/fnmode file:
echo 0 > /sys/module/hid_apple/parameters/fnmode
I updated the firmware on my device, and this seems to have resolved the issue I was experiencing.