The Photos app does not detect faces in photos synced from my Mac.

Have the photos been synced manually from your Mac syncing with the Finder? Or are you syncing the photos with iCloud?

It does not make much sense to scan the photos synced from the Finder or iTunes for faces, because each new sync will replace the previously synced photos by the new sync.

The photos synced with iCloud should be scanned for faces however. my iPhone is scanning the photos synced with iCloud for faces, no problem.

How are you syncing, with the Finder or with iCloud?

I don’t really know what you mean by “copy synced photos to library”, there is only one photo library which contains all photos. When I duplicate a synced photo, then this photo duplicate is scanned. Faces are recognised.

stanoman28 wrote: … there is only one photo library which contains all photos.

As léonie was pointing out, that's not quite right. Some pictures that you see in Photos aren't actually in the Library. That's why they behave so oddly.

When I duplicate a synced photo, then this photo duplicate is scanned. Faces are recognised.

That's great! I think that you've figured out what to do! Duplicating is pretty much the same as what léonie suggested. She wanted you to take those pictures that don't actually belong to Photos and re-enter them, and, surprisingly to me at least, "Duplicate" does exactly that.

I have the exact same problem. Would be great to get a fix for that.

As a test, try to copy a few of the synced photos into your Photos Library on the iPhone. Will Photos now scan the synced photos, after they have been properly imported to Photos?