I tried to reproduce your issue, but it seems to be working for me: http://plnkr.co/edit/AuACV3SZRPgEqGBWpvkl
I hope you can identify the cause using this code.
You may want to make sure contents of selectedDesignAreaId
and designArea.id
and such.
just putting {{ selectedDesignAreaId }}
into your template would ease your debugging
process. (IMO, it is even handier than using dev console.)