Maybe I'm looking at this wrongly, but it appears to me you are looking for 2.8 while the lens is not a 2.8 lens so it won't have an aperture of 2.8, meaning the search was correct to not return any results.
But the real problem is that you have “Whole Words” ticked. Since a dot is a word separator, it will turn 2.8 into two words. Causing the search string 2.8 to never match. Even if you had 2.8 in the searched metadata…
Instead of ticking “Whole Words”, select “Grep” and use
\b2.8\b as search pattern. This will give you what you are looking for