I conducted two “identical” searches, one using an index (Anywhere) search and one using a smart folder with the same selection criteria for each (two tags and a word).
The searches identified the same files, but the word was highlighted only in the index search.
That’s how it’s designed to work. The highlighting in the records list and viewer uses the text entered in the search field. Selecting different sources in the sidebar (Records, folder, tag, smart folder, Trash) is not considered to be a search—it’s just changing the scope of what’s viewed.