Names of faces disapeared

Which version of Photos and MacOS X are you using? The handling of the named faces changed considerably between the differnet versions of photos.

Possible reasons for vanishing faces names may be, that you accidentally merged two persons.

Or you may have hidden the album of a person in Photos 1 or 2.

See this help page: https://help.apple.com/photos/mac/1.2/?lang=en#/phtad9d981ab

https://help.apple.com/photos/mac/1.2/?lang=en#/phtad9d981ab

Hide all photos of a person in the People album

You can hide all photos of a person in the People album.

Click People under Albums in the sidebar.

Control-click the person, then choose Hide This Person.

The photos are hidden from the People album, but not removed from your library. To show a hidden person in the People album again, scroll down and click Show Hidden People, Control-click the person you want to show, then choose Unhide This Person.

But it seems you are fairly certain that I have to redo the renaming?

I had to name all people again after the High Sierra upgrade, yes,, but I am using iCloud Photo Library, so our systems are not comparable. There is no Apple support document explaining how the High Sierra upgrade will handle the already named faces, only a promise that it has been improved "more accurate groupings"

https://www.apple.com/macos/high-sierra/

The People album gets even better with faces.

Easily find your favorite subjects in the People album with larger image thumbnails and more accurate groupings. And when you use iCloud Photo Library, your album stays up to date on all your devices.

For me, it was not much of a problem to rename all faces, because I had previously tagged all photos with keywords with the faces names. I di this, because in the previous system versions the named faced did not sync with iCloud, and so I could use the keywords to search for persons on other devices.

The keywords helped me to create smart album for each person to quickly add the names back.

This issue has been discussed before:

iOS 11 photos person has disappeared

Photos app keeps rescanning, deletes names

As of today, I have filed two bug reports which were not dealt with; this bug has been introduced with iOS11 at an early beta stage and appears to affect people who are syncing their photos from a Mac via iTunes only, as it is not occuring on my mothers iPad which syncs its photos from iTunes for Windows. None of this has to do with iCloud Library (it only happens to people syncing locally); it does not have to do with High Sierra per se as well. There has not been a fix in 11.1, 11.2 or 11.3. Naming all those people again will not help you at all since once you plug your device in for charging, some bug will trigger Photos for iOS to erase all names assigned and scanning for faces anew. I did actually consider an eMail to Craig Federighi about this issue since no one seems to care.

I don't know exactly. I have 10.13.3, when was that released? I am usually rather quick to update.But how on earth can an upgrade make that happen? Have they not tested the upgrade?

I did not mean the 10.13.3 incremental update. The upgrade from 10.12 to 10.13 introduced a completely new face detection. So photos had to scan all photos again for faces and not all previously detected people names were carried over. I found thousands of new unnamed faces in my library, that had been missed by the previous versions or found in a different location. Additionally, the faces detected on the iPhone and iPad also synced to the Mac with iCloud Photo Library, also the faces detected on my other Macs. Are you using iCloud Photo Library?

Naming all those people again will not help you at all since once you plug your device in for charging, some bug will trigger Photos for iOS to erase all names assigned and scanning for faces anew. I did actually consider an eMail to Craig Federighi about this issue since no one seems to care.

If you are syncing the photos with iTunes to your device, it is the expected behavior. Whenever you sync again with iTunes all previously synced photos will be replaced by the new sync. For the newly synced photos you will have to name the faces again. There is no help for it.

You are mistaken. This bug occurs even if you are not syncing via iTunes in the meantime. Sync once, name those people, charge your device once or twice for a suitable amount of time and (bang!) your names are gone.

Trust me, I know my way around bugs and this is NOT expected behavior; it was introduced with iOS 11 or High Sierra, probably as a result of those two not playing together as intended. It doesn't happen if you sync via iTunes for Windows ⚠, it did not ever happen on iOS 10 (used it for a year) ⚠ and since iTunes keeps a cache of your synced Photos library, it should not happen at all, since only photos which have been modified on High Sierra since the last sync will be transferred to photos for iPad/iPhone again ⚠.

Repairing Photos Library for Mac does not help, emptying iTunes Caches does not help and rebuilding Photos Library on the Mac from scratch does not help. This bug has plagued me since last year and I haven't found a solution as of today.

You are mistaken. This bug occurs even if you are not syncing via iTunes in the meantime. Sync once, name those people, charge your device once or twice for a suitable amount of time and (bang!) your names are gone.

I did not deny that. It happened on my devices too, and I am syncing no longer with iTunes. My reply referred to your sentence "Naming all those people again will not help you at all since once you plug your device in for charging, some bug will trigger Photos for iOS to erase all names assigned and scanning for faces anew." Syncing an iPhone with iTunes on a Mac will erase the named faces for sure on the iPhone or iPad for the previously synced photos, because they will be replaced by new copies. Enable the iTunes Preference "Prevent iTunes from syncing automatically, when a device is connected" in iTunes > Preferences > Devices.

This sentiment is incorrect. It were correct if you tried to sync a different library or via a different PC/Mac. If you are syncing the same library from Photos for Sierra/High Sierra (etc.) from the same device as before (which is the case), iTunes will only add or change Photos which have been added/edited since your last sync. Under iOS 10, this did NEVER affect names assigned to people. The expected behavior for Photos on iOS in this case is to scan only the Photos changed or added for new faces. The issue that Photos for iOS keeps deleting names/people from the Album "People" – even if I haven't synced from my Mac for weeks – does not have to do with the option "sync automatically", since my iOS device wasn't even plugged into my Mac for weeks, yet keeps deleting names while charging over night.

I really appreciate your effort in trying to solve this issue, yet I urge you to read the previous threads and posts carefully. You will see that your solution is not related to the issue which is presented here and elsewhere.

I did. You mentioned iTunes syncing first and changed the direction of the discussion.

The original post is about faces vanishing in a photos library on a Mac and we tried to find likely reasons for that.

Did you just upgrade to a new system version? The High Sierra upgrade changed the face recognition and face detection algorithm. Some faces need to be named again.