In xfwm4-settings-dialog the organization of the buttons uses DND to drop/add buttons to the title bar. This DND action is only working when the dialog is not plugged inside the settings manager.
Reproducable here. Assigning this to me. I'll have a look at it later today.
Assigning back to Stephan who initially wrote the DND code. I have no idea what's wrong with it.
Increasing priority.
*** This bug has been marked as a duplicate of bug 4524 ***