People will finish updating when Photos is in the background

From this article:

Launch the Photos app on your Mac.

Select the picture that has the person's face you want to create a profile for.

Click on the Info button on the top right side of the Photos window. It looks like an i with a circle around it.

Click the Add button next to Add Faces.

Relocate the tag circle to the face of the person you want to create a profile for.

Click on Click to Name.

Name the person. If the name is in your contacts, it will appear in the suggested list.

Click the Close button in the info window when you're done.

Click on the People album in the sidebar under Library.

Select the new People profile.

Click on Confirm Additional Photos to see if the Photos app has automatically registered any additional pictures with that person in it.

Had exactly the same problem in macos Catalina v10.15.2 and Photos v5.0 (121.17.170).

I had tagged one photo with one new person and I waited days for more pictures of that person to be discovered.

Hopefully this might help:

After manually tagging a few more photos with that "new" person, came back to People and clicked on the person's face, then scrolled all the way down and this time the "Confirm Additional Photos" link was showing up.

I looks like maybe the face matching algorithm needs a few photos manually tagged before it can work?