I have face-tagged our 50,000 photos. However, it seems that many of the face tags are being auto-deleted

What's always happened to me in Photos is that when Photos forgets a face tag and I re-tag it, the program creates a new person, rather than seeing the person who was formerly tagged.

This happened to my library too, after the upgrade to Catalina.

Catalina seems to have changed (or improved ) the face detection algorithm. So all faces are scanned again, and do to the new algorithm some faces are detected in a slightly different position or not at all. Photos has used the previously assigned names in many cases, but is waiting for a confirmation, if the the faces are not perfectly identical. The previously assigned faces remain hidden, if Photos could not confirm them on its own. If we now assign a face on our own, the person would appear twice in the photo - as the hidden face and the new face we just assigned. Photos is treating them as different people, because Photos 5 does not allow us to name two faces in a photo with the same name. This is probaböy a bugfix to prevent syncing errors for faces across devices, but if Photos assigned a name to the wrong face, we cannot assign the name to the correct face, before we removed the name from the wrong face (which is difficult, if this face is still hidden).

It helps to use "Confirm additional faces" for all People albums, before we start to name faces manually.

May I suggest an addition step in order to protect your tagging? Every time you add a face to a photo add a keyword for that person. Then it's very easy to group all photos that include that person with a Smart Album, Keyword is "XXXXXX".

To keyword existing face tagged photos go to the People section, click on a face, select photos, select All and use the Keyword pane (⌘+k) to add the keyword for that person.

The face tags of people in Photos in my experience is buggy and utterly unreliable. I recently upgraded to Catalina on a new Mac Pro using an existing library, and found that many of my 61,000 photos have been untagged. So I took Apple's advice and used the repair library tool. That made the problem worse. Now many of my photos have received shiny new "unnamed" tags, which are overlaid on top of their previous tags. Because Apple in its infinite wisdom doesn't allow mere users to delete face tags, I guess those "unnamed" double-tags will be there forever, or until the program accidentally deletes them again. I have spent hours fighting this program, and I wish Apple's engineers had spent half that time debugging it.

Sorry I know that's not the same thing as helping, but I just wanted to let you know, disappearing face tags is quite common and definitely an integral part of this awful program.

I still use this extended workflow to identify my photos.p

1 - download each shoot to a folder on the Desktop whether it's from a regular camera or phone camera.

2 - name the folder with the date taken and a very brief description.

3 - with a file renaming app like NameMangler I rename the image files with the date taken, brief description and a padded sequential number at the end of the file name.

4 - save a copy of the folder to an external HD.

5 - import the folder into my family photos library with the option to maintain folder organization.

It may seem like a lot of work but I never have a problem finding a photo from a specific event or date.

Thanks! I actually already do most of what you do, surprisingly, aside from renaming originals.

Regarding the face tagging, one reason it's important is (unless as you suggest the photos also receive a keyword with the person's name) that you may forget years later which pictures are out there with the person in it. When my mother-in-law died, it was very frustrating prior to her memorial not to be able to find the photos of her which had been untagged by the program. Very frustrating.

One other question, off-topic for the face tag issue but when you "maintain folder organization," does that mean that you're using a referenced library? I did that for years, until the associated problems (incorrect thumbnails, corruption) made me switch.

Can I ask a question? What's always happened to me in Photos is that when Photos forgets a face tag and I re-tag it, the program creates a new person, rather than seeing the person who was formerly tagged. As a result in People I now have many duplicate people, I think because the program is accidentally breaking the link between the image and the tag which had been created. Does any of that happen to you?

Thanks! Interestingly, this is an issue I've had in Photos for years, going back to at least High Sierra.

So léonie, you're recommending as a fix to go to "Confirm additional faces," but does this solve the duplicate face tag issue?

Sorry for hijacking your thread, sdesselle! At least it's basically all face tag related...

If you're on 10.15, then this is a Photos query, not an iPhoto one, correct?

I'll ask this be moved to the correct forum.

Thanks for the tip! More work to do to help Photos work as intended, I guess.

Photos V5.0 has a way to go to fix it. I did a repair on my primary library and it cleared out all of the faces in the library. Good thing I had a Time Machine backup I could restore.

Thanks! What's the way to fix it in 5.0?

I also have a backup I could restore, but I'd never be able to repair the library, if needed, without recreating the problem, right?

I wish I knew. Currently I'm keyboarding all new photos to the library so I'm covered that way.