I got a similar problem : Notebook and external monitor.
When I put the monitors "side by side" the "Primary monitor" setting works OK.
BUT if I place the secondary monitor over the laptop, the "primary monitor" setting is ignored and the above monitor is always primary, whichever setting I choose
BTW I did not succeed to find how contact Xfce devs or buglist
I got a similar problem : Notebook and external monitor.
When I put the monitors "side by side" the "Primary monitor" setting works OK.
BUT if I place the secondary monitor over the laptop, the "primary monitor" setting is ignored and the above monitor is always primary, whichever setting I choose
BTW I did not succeed to find how contact Xfce devs or buglist