You may want to provide more information of the copying process (not the details of PlotSettings), such as which drawing (source, or destination) is open in AutoCAD editor? or one of them is opened as side database (then which one, source or destination)? or both open in AutoCAD editor? or both opened as side database?
For example, if the source drawing is currently open in AutoCAD editor and the destination drawing is opened as side database, have you tired to set HostApplication.WrokingDatabase to the side database?