Boolean searches not working

If I do a search with a specific phrase, the search results yield nothing—even though I know the phrase appears in multiple files.

 

For instance, when I search a folder with Microsoft Word docs for the exact phrase "rational brain," no files come up. 

 

I am able to get results with these searches:

1. "rational" AND "brain" yields multiple hits
2. rational AND brain (no quotes) yields multiple hits
3. rational brain (no quotes) yields multiple hits

 

BUT, the search that would actually provide me with the files I need—"rational brain"—yields nothing.

 

Of note, I know for a fact that "rational brain" appears in multiple files. 

 

I'm on a Mac Air 15-inch, M2, 2023, and doing the searches from the Dropbox website using Chrome or Safari. My subscription is Dropbox Plus.

 

Please help!