I think it's a new "feature". The action of creating a new document clears the "copy" or "cut" cache.
Office 2003 doesn't do this.
I abandoned New Office (2007 at the time) and reinstalled Office 2003. I couldn't handle the OTT array of drop-downs and sub-menus when all I wanted was simple buttons to do stuff. I gave away the New Office to the kids next door, who love the eye candy.