Actually, I had this problem where I accidentally selected a large amount of untagged faces in photos and chose "Don't Show Again".  I searched through all the similar posts and searched for pictures.pd6, pictures.pd4, and other mentioned files to delete with no success as they were not on my machine.  The solution was actually super simple.  I simply right mouse clicked on my photo folder with my pictures and chose "remove from gallery".  This does not delete the photos, only removes it from the Library.  Then I closed Photo Gallery and reopened the app.  When you reopen, right mouse click on the top folder "All Photos and Videos" and choose "Include a Folder in the Gallery".  It will then rebuild the facial recognition of untagged faces.  It did not lose any previous tags I had entered before as far as I could tell, but it did start showing me all the photos with faces that needed tagging.  It is still building as I type, but as it did say there was nothing to suggest, and I now have hundreds showing, I believe it is working.  Hopefully, it will work for you too.