Are you positive it's the NVDA search you're getting there?
On the Gmail webpage, if I hit NVDA+Ctrl+F I do get the NVDA search, and it finds what is shown on the page itself (as in if the word is either in the sender's name or the subject of the message) but it does not search the content of the messages themselves (or at least doesn't appear to, which is what I'd expect). If I hit F3 after an NVDA Find, what comes up is the regular "Gmail find," which is different in behavior even from a typical webpage find as it searches not only what you can see on the screen but the content of the messages themselves.
There could be some sort of "mixed search mechanisms" situation going on here.
Brian - Windows 10 Pro, 64-Bit, Version 1903, Build 18362
The color of truth is grey.
~ André Gide