Are the faces that keep vanishing faces you have added manually?
The manually assigned faces circles are not as stable as the faces that Photos has automatically detected Manually added faces can vanish, when we install a software update that is improving or changing the face detection algorithm. or when we download our photos again from iCloud.
As a precaution to save the face names, I am always adding a keyword with the name of a person, when I tag a face in the People album. This way I will always find all photos of a person, even if the face has reverted to "unnamed" and can use a smart album to find quickly all photos where the name has been lost.
See: How to use Keywords to Back Up the Named Faces in Photos and Improve the Search for People … - Apple Community