LIGHTROOM: Keyword claims it's assigned to more images than are in the gallery

I have thousands of galleries I've created with Lightroom with perhaps a million images, averaging 500 per gallery. I just ran into a new problem whereby after importing and sync'ing metadata, I show a keyword that claims to be assigned to exactly twice as many photos are are in the gallery. when I select all images, it shows a dash indicating that not all images are assigned this keyword. Clicking to force it to assign and perhaps recount things does nothing. Using the metadata panel to find only images with that keyword also reports twice as many as are in the gallery. Any thoughts on whether this should be a cause for concern?