Null sinks volume and mute state can not be set in Pavucontrol
For some reason I was never able to set null sinks volume and mute state in Pavucontrol. I did not try to test yet through the native PipeWire API.
Although this should be fixed eventually I have to say that the current situation is very good for PulseEffects. In Pulseaudio there are games and possibly other applications that decide to mute our virtual sinks or changing their volume. That is super annoying. So in a way I am happy with this bug.