I created a smart group where I wanted to show every file in a folder that did not have a Red or Magenta label. It worked for the Red labels, but I noticed the Magenta labels were still showing up. So I edited the smart group, and Red was the only label there. So I added Magenta and saved it. All the Magenta files were then gone, but the Red ones popped up! So I edited the group again, and Red was gone.
Apparently DTP won’t let me add more than one label to a Smart Group. I tried changing it to ‘is’ instead of ‘is not,’ with the same results.