(Sorry for the delay - dealing with sick pets)
I'm using 7.1.3. I started with 7.0 and updated through every upgrade version. I just re-checked my images and settings to be double-sure, and per what I see there should be no problem. Each of about 90 images (out of ~3000) has one keyword, and my smart album is set to include all images that contain that keyword. And yet the smart album only includes a handful of images. One catalog, all images managed.
One thing PhaseOne support had me do was verify the catalog to look for errors and fix if found. I'm finding that if I run verify I'm as likely to get errors as not. I tried it again just now and it needed repair. After repair, the behavior was unchanged. When support heard about this earlier, they suggested making a new catalog and re-populating it with the same images. I've done that, and it changed nothing.
I really like the image quality and tools in C1 pro, but their database management leaves a lot to be desired IMO.