Volume/Media Keys

I've been having issues with keyboard shortcuts for volume keys. They work when I manually set new shortcuts but stop working after reboot or new session.

As a workaround I can fix the issue and get them working by running at startup:

killall mate-settings-daemon && mate-settings-daemon

Afterwards the keys function correctly so I've created a custom startup script.

Does anyone know the cause of this or how I might permanently fix this issue?