macos – What are the requirements for Photos to start a facial scan?

I've recently built a large library of photos and, now that almost all of the content is available, I'd like to start filling in the people album. Photos indicates that it "will continue to scan the remaining photos when you do not use the application and your Mac is turned on". Therefore, I let the application run in the background (opening and focusing in another window), telling my Mac to never sleep, plugging it in and leaving it all the way night.
This, however, does not seem to work; indeed, the number of digitized faces seems to be decreasing (yesterday, 57,807 photos had been digitized and only 57,381 left). I seem to meet perfect conditions and yet nothing happens. I should perhaps mention, though, that I had it a little each time, but not in a way that should affect it, I hope.

  • The first two nights, I stopped the photos
  • The third night, I left concentrated Photos.
  • And tonight I left Mission Control.

The third night I left Photos
concentrated and tonight I left Mission Control running.

What is causing this problem and what am I doing wrong?
If this can help you, here are the specifications of my system:
My system specifications:
Mojave