However, I had a similar issue (primarily in searching for a specific note) on macOS High Sierra during quite a few of the betas. Logging out and logging back into iCloud solved it there. Flawless now with the final release of High Sierra.
The issue is that I have in-app searches disabled in Siri & Search. For example, I don't want to type in "Sp" and get every email that has Sp in it etc. I want Sp to bring up app names only. But, by disabling the in-app search for Mail, I have no choice - and it disappears completely, unless I type in the entire app name.
(I do not want content from Notes to appear in search....)
The issue is that I have in-app searches disabled in Siri & Search. For example, I don't want to type in "Sp" and get every email that has Sp in it etc. I want Sp to bring up app names only. But, by disabling the in-app search for Mail, I have no choice - and it disappears completely, unless I type in the entire app name.
(I do not want content from Notes to appear in search....)
Unfortunately it seems as if Apple changed that behaviour in iOS 11. Unless you have in-app search enabled you will not even be able to search for that app unless you type the full name.
Only way to mimic almost the same behaviour as before is to disable Siri search entirely.