I have two sound cards installed. When I run two instances of xfce4-mixer the Wannabe Master setting is ignored in the second instance. The result is that no matter which Wannabe Master I manipulate the volume changes always for the first one. If I go the the properties and change Wannabe Master to i.e. PCM and then back to Master it starts to work ok.
How do you start xfce4-mixer? Do you pass the device name as argument? It has no way of knowing which device you want this time around otherwise.
So you mean it doesn't save the Wannabe Master the first time you edit the properties? I mean you have to set the properties for the new plugin instance once (when adding), right? And that's where it doesn't work? If so, I might have fixed that now.