When I manually add names to photos the system acceps them but later erases them so many y photyos cannot be put int proper people files

When do the manually assigned names vanish? Out of the blue or after a system upgrade?

When we manually mark faces that Photos did not recognize as a face, the manually assigned faces may vanish, whenever a system update adds improvements to the face detection and face recognition. Then Photos will scan again for faces and when the manually assigned faces are in conflict with the results of the new faces scan, the manually assigned faces may be discarded.

I am taking the precaution of adding the names of the people also as a keyword to the photos. The keywords will usually survive an update or even the migration to a different app. and on my Mac I can check for missing named faces of a person with a smart album, for example searching for "keyword is Mickey" and "Person is not Mickey".