Apparently, Outlook uses Spotlight for indexing and search functions. So even though the search is inside Outlook and it looks like Outlook is doing the searching, Outlook is really using Spotlight. Spotlight is supposed to index all readable content on
your Mac.
For some reason, mail in Outlook stops getting indexed by Spotlight, and thus stops appearing in search results in Outlook (because it's not in the Spotlight index).
Deleting the mail account in Outlook, then re-adding and waiting for the mail to get indexed is the only thing that has worked for me. Unfortunately, it doesn't work permanently; after a while, some mail doesn't get indexed again and isn't searchable. But
it worked for me for a while as a temp hack fix.
If you're running 10.14, it's an Outlook bug, and so searching in Outlook (regardless of the health of the underlying Spotlight index) will only work intermittently, if at all, until Microsoft fixes the problem in Outlook. As an earlier poster mentioned,
the problem is documented here, has been an issue since November 2018, and is not yet fixed.
Until they fix this, I've abandoned all searching in Outlook and instead I'm using the web-based client (I'm on O365) to search. I'd recommend using whatever web-based client you have to your email to do the same, or risk inaccurate search results (usually
stuff is just missing).