How do I find out which folder an app is in in iOS 11?
It works if you tap on it in the list of hits.  E.g if I search for "Pages", tap on my top hit in the highlighted list of hits, it shows me the app, Pages and to the right is the folder name "Office" (in my case).  Or "Stocks", tap the top hit, and I see the Stocks App, and it is in the indicated folder "Little Used".

Source: https://discussions.apple.com/thread/8102281 

Also confirmed on my own iPhone 7 Plus ios 11.3.1