Assigning mutiple Materials to one Polysurface

Hi.
I found out it is possible to do this in Rhino 6.However I am only able toassign a different material once to one face only…

If I Drag-Ctrl-Shift Materials to other faces after the first subsurface assignment, it always selects the whole object.
Could it be a bug or is this my fault?

// if preselect everything and then apply materials via the right click menu it works… so its something about the drag shift method i guess…