mappings ignored when a gnome menu is open

I've mapped the forward/back buttons on my mouse (Logitech MX Ergo) to increase/decrease the system volume. This works without any issues, *unless* a Gnome menu is open (e.g. the tray icons menu or the Activities overview). In that case, it appears that the mapping is ignored as the system volume does not change upon pressing the mapped keys. Instead, the default forward/back action is performed. I can confirm this by opening the tray icons menu over a Chrome window and subsequently clicking the forward or back button on the Chrome window: Chrome navigates to the next or previous page respectively.

This is my .xbindkeysrc:

1 # volume down
2 "amixer -D pulse set Master 10%-"
3 b:8
4
5 # volume up
6 "amixer -D pulse set Master 10%+"
7 b:9

I'm using xbindkeys version 1.8.6-2 on Ubuntu 20.04 LTS