Update: this feature has now been added to Google Photos! You can select any face and either create a new person or map it to someone in your library.
After waiting for a couple of weeks, the last ~two years worth of photos were analyzed correctly and new faces have been added to the library. However photos taken since before 2017 have not been autoprocessed after one month of waiting. Therefore I've had to trick Google Photos to think that I've uploaded new pics:
- Sync your photo folder from Google Drive to your local machine
- Move the photos from the Google Drive folder to another folder and wait for Drive to sync
- Move the photos back and wait for Drive to re-uploaded them back to the cloud
A few minutes after re-uploading the photos they should be correctly mapped to your faces library. After a couple of weeks Google Photos will parse any new people and add them to your faces library as well.