Completely disable the volume OSD in Gnome
I've managed to find an alternative solution to this problem that bypassed the Gnome volume OSD completely, though it might not work for all.
I've simply removed the Gnome keyboard shortcuts for the volume control, and instead created some custom keyboard shortcuts that run a command. The commands I've used are simply:
Volume Up pactl set-sink-volume 0 +5%
Volume Down pactl set-sink-volume 0 -5%
Mute pactl set-sink-volume 0 0%
It's not ideal, as I can't unmute with the mute key, but it solves the immediate problem.