On a HP Elitebook 840 G6 laptop, both the backlight brightness keys (F3 and F4)
toggle microphone mute state instead of doing their intended thing.

The good thing is that other keys, such as mute, volume and the actual
microphone mute key (F5, F6, F7, F8) work as intended.

No messages appear in dmesg --follow or journalctl --follow when pressing these
keys.

In evtest, the brightness up and down keys appear as the same identical
sequence, which is also a common prefix of the actual mic mute key:

Common prefix:
type 4 (EV_MSC), code 4 (MSC_SCAN), value 81
type 1 (EV_KEY), code 190 (KEY_F20), value 1
-------------- SYN_REPORT ------------
type 4 (EV_MSC), code 4 (MSC_SCAN), value 81
type 1 (EV_KEY), code 190 (KEY_F20), value 0
-------------- SYN_REPORT ------------

Actual mic mute key (unique suffix part):
type 4 (EV_MSC), code 4 (MSC_SCAN), value 42
-------------- SYN_REPORT ------------

Kernel: Linux 5.14.6-1-default

I found similar reports about these keys not working:
1. https://bugzilla.opensuse.org/show_bug.cgi?id=1105997
2. https://bugzilla.opensuse.org/show_bug.cgi?id=1160911

The first one was about not showing it correctly, but the last one says:
    Update: The issue is gone in the latest BIOS update issued by HP.
        Version: R70 Ver. 01.04.05
        Release Date: 02/18/2020

That seems to be the same version as I have (according to lshw):
*-firmware
    description: BIOS
    vendor: HP
    physical id: 1
    version: R70 Ver. 01.04.05
    date: 02/18/2020
    size: 64KiB
    capacity: 32MiB
    capabilities: pci pcmcia upgrade shadowing cdboot bootselect edd
int5printscreen int9keyboard int14serial int17printer acpi usb smartbattery
biosbootspecification netboot uefi

I haven't tried updating BIOS, but it doesn't look easy.