This one, I can reproduce at will. Using a dual monitor configuration under Windows.
1. Launch LR. Go to the Library module. Main screen in Loupe mode, secondary display in Grid mode. 2. Select a smart collection. 3. Select an image. 4. Exit LR. 5. Relaunch 6. The main display shows again the lastly selected image. The secondary display says "No photos in selected smart collection". 7. Now if you select another smart collection and go back to the previously selected one, all the images of the smart collection are shown on the secondary display.
Sigh! Yet another bug introduced with version 7.3.
I can duplicate what you are seeing here Patrick on Mac 10.13.4. When I have Color Label selected as a Smart Collection criteria I get the the message. When I remove Color Label I get a normal grid in the second display view. I will log a bug on it. Thanks for reporting and providing the additional detail.