This is so friggin obscure now. I had to read this Apple Discussions thread a few times. The problem is there are two types of Spotlight search in iOS, and now only one of them shows a containing folder name.
When you type an app name in Spotlight iOS does predictive search first. You don’t tap the Search button. In iOS 11-12 the predictive search result UI doesn’t show the name of the containing folder.
To see the containing folder you need to do search-search (plain old search), not predictive search. Type the portion of the app name that shows your app in predictive search, but don’t tap on the app. Instead tap on the Search (blue) button. That does proper search, and now the containing folder shows next to the app name.