All the data about pictures in Photos is contained in the database. Spotlight uses the database. (Now it does-- it used to ignore Photos.)
When I search for Richard, I get 1400 hits. But I use the main search field just for quick searches. Most of the time I would use a Smart Album, and for people I use keywords.
The Person albums we've been talking about are rather ephemeral, as you've seen. You can't really depend on them finding every instance, or even being exactly the same in the next OS. Many of us use the Person Album mostly as a help in adding keywords to pictures. In the Richard person album I selected all and added "Richard" as a keyword to them. Other pictures I find that have "Richard" in them, but just the back of the head, maybe, I add the Richard keyword, but that picture may never show up in the Richard album. Nonetheless, I can find it when I want. This applies to everyone I want to identify. With keywords and Smart Albums, I can find pictures that have Richard and my daughter together, and without in-laws. Always a good thing.
Spotlight, by the way, works the same way-- it builds a database for what it can find anywhere. It's busy hunting through our files just like Photos is busy hunting through our pictures.
The coolest use of the Face names is with those family pictures with a whole bunch of people you've never met.
