I had the same problem after the upgrade to Catalina. The Pople album of my husband got mixed up with my people album. I could track this down to a few photos showing me and my husband together in the same photo. The new faces scan after the upgrade had only found on face in one of the photos - the face of my husband. Previously the faces scan had detected my face but not the face of my husband. What seems to have happened is that after the new faces scan my name has been assigned to the face of my husband as the faces were close together in the photo. I saw similar results for other couples.
To fix this, it helps to use temporary different names for the two mixed up people. Do not use the same name as before - try a new nickname, that is not in your Contacts. Photos will remember that the two mixed names ar referring to the same person and instantly try to mix the faces again.
I would first assign the correct original name as a keyword to the photos that need fixing, so you can find them quickly by searching for the keyword with a smart album.
Then reject all wrong names with "This is not ..." in the People album.
To add a new name collect all photos of person 1 in a smart album with there "keyword is ...".
Now view the photos in this album enlarged and rename the faces with the new, temporary name.
Do the same for the second person.
Once the face names are correct and stable you can try to use the original name again, but I would make a backup copy of the corrected library, just in case Photos has a very long memory and still considers the two contacts to be the same person.
See: How to Correct Merged People Albums - Apple Community
