No audio notification when toggling microphone (source) mute/volume from pactl

When controlling the microphone mute/volume from the command line e.g. with

pactl set-source-mute @DEFAULT_SOURCE@ toggle
pactl set-source-mute @DEFAULT_SOURCE@ 1
pactl set-source-volume @DEFAULT_SOURCE@ 80%

No notification overlay is shown. This is opposed to

1. Controlling sink mute/volume with pactl does show the notification overlay.
2. Controlling the microphone mute with the multimedia (fn)-keys does show the notification overlay.

Tested with xfce4-pulseaudio-plugin 0.4.3 / Archlinux.

This is the notification I am referring to: