On my Librem 15, I have to press the "Volume up" and "Volume down" keys (fn + arrow up / down) twice for it to take effect.
This is not specific to the audio UI because if I assign those keys (fn + arrow up / down) to a keyboard shortcut through the GNOME keyboard settings, the event triggered by this shortcut only gets called after the second time I press the keys.
No other "fn" key on the keyboard behave this way.
I am having this issue on a Librem 15 v4 running a basic BIOS (No Coreboot) and PureOS with the latest updates (at the time of posting this issue).