The names we assign to faces are not very stable. Whenever Apple improves the face detection or face recognition, the photos need to be scanned again for faces, and if the improved, new methode will detect the afce in a slightly shifted position or no longer at all, the face may now be showing as unnamed or not at all. also the iCloud syncing of faces between devices can cause conflicting face names.
And none of the faces I have tagged manually by adding a circle around the face has ever survived a system upgrade. I have to tag the same faces over and over again. Only the faces detected by photos have survived the system upgrades. See: How to use Keywords to Back Up the Named Faces in Photos and Improve the Search for People … - Apple Community
If you are now seeing some previously faces as unnamed, try to use the "Confirm additional faces" dialog. If Photos is no longer sure, that the face it has found is the same person, it will be waiting for a confirmation. The request to confirm additional faces may either appear at the top of the album of a person or among the links at the bottom of the album of the person or pet. Before we add names it is better to clean up all the names, that Photos is suggesting but not yet showing, because it is not sure and is waiting for a confirmation. This will help with the cases, where the face recognition in Photos is wrong.
Also, I recommend to add the names as keywords in addition to the names below the faces. The keywords are very stable usually don't get lost during a system upgrade. And they can be exported when we share a photo or save it to the Finder and will be found, when we search for a certain name.