Ok, this seems to be a new feature of the 24.04-1.0 release. This seems to be desired by others, but hinders function of my equalizer app. An option to disable/enable this behavior in the system settings would be nice.
Interestingly this does not happen, when moving the sink inputs with e.g.: pactl list short sink-inputs | awk '{print $1}' | xargs -I {} pacmd move-sink-input {} equalizer and back with pactl list short sink-inputs | awk '{print $1}' | xargs -I {} pacmd move-sink-input {} sink.primary_output.
But to implement this function into my app and consider all the different eventualities will be very complicated, i think. Changing the default sink is much easier.