Photos has unnamed people that do not appear

It is the expected behavior in Photos 3.0 on High Sierra. Is that the version of Photos you are using? Photos is trying to keep the People album short and and will only add new, unnamed faces to this album, if it recognizes the same person frequently. From experimenting, the limit for frequently seems to be in more than fifteen different images. So it can take a while, until a new person will be added. To speed this up, add a name directly on one of the photos of new people, that you want to see in the People album to start the album. Once the album has been added, open it and scroll down to "Confirm additional faces".

This limit is not mentioned in the Photos user Guide for Mac, but in the iPhone user guide.

https://help.apple.com/iphone/11/#/iph9c7ee918c

Photos scans your photo library for people’s faces and adds the most frequently found faces to the People album. You can assign names to the people Photos finds, add people who appear less frequently, mark people as favorites, and view memories that contain specific people. The more photos you have, the longer the first scan takes.

This will prevent your People album from decluttered by the faces of each and every stranger detected in the background of a photo. In previous versions of Photos you could remove automatically detected faces by rejecting them, but this is no longer possible in Photos 3.0 on High Sierra or Photos on on iOS 11.

I found it in a post of another user who tracked down the magic 15 in one of the databases inside the Photos Library. I'm trying to find this discussion again, but searching these forums is a bit difficult.

Added: Found it: taffjones https://discussions.apple.com/thread/8011768?answerId=32553855022#32553855022&ac_cid=tw123456#32553855

There is a hidden preference for

~/Pictures/Photos Library.photoslibrary/private/com.apple.photoanalysisd/VisionService/faceWorker State.plist PersonBuilderLastMinimumFaceGroupSizeForCreatingMergeCandidates