This is what I see . . . Some confirmed faces are greyed-out. Clicking on 'photos' shows the correct photo and the 'tag' is correct.
How can I get Elements Organiser 2019 to remake the missing faces?
I believe what you are seeing are faces that were tagged by you in an earlier version of Elements without face recognition, and they were converted to People tags. So, since the faces have not (yet) been face-recognized, the faces will show up with a generic thumbnail.
I am more curious about the blue glass thumbnails. I assume these have been confirmed as different People.
"How can I get Elements Organiser 2019 to remake the missing faces?"
You will need to name the faces again in the UnNamed People room.
Thank you for your advice. The images in question had been tagged using older software. The thumbnails were confirmed as being the correct People. I followed your advice though and renamed the faces again . . . That fixed my problem.