When you run 2 or more instances and click the categories radio buttons, the treeview will refresh in each of the different instances.
Created attachment 2906
No need to send a notify for category
Looking at the code, g_object_set("category", ...) calls _xfce_appfinder_window_set_category and this particular function calls g_notify(..., "category").
Something I now learnt is that notify are working for IPC, but this is not something we want here. The patch removes the g_notify call.
That fix won't work properly, the notify is needed for the xfconf binding to save the new setting. Instead I dropped the binding and used normal set/get in master.