However, the Frequent folders section was showing up and seemed to work as usual. In fact, the Recent files section was missing. Recently, I noticed that the Quick Access window was not showing recently opened files. Haven't noticed this kind of behavior with the other mapped drives. Quick Access, with default settings, displays up to 10 frequent folders and 20 recently opened files. Most of the time something happens, don't know what, and all of the other folders, including the one for the LOB app appear again.
Sometimes if you just click on one of the other mapped drives and go back to the F: mapped drive all of the folders now appear.
But then it will eventually resolve itself. When I look at that mapped drive (F:), sometimes it will have a red X on it as if it is disconnected but when I click on it, it opens but only shows 1 folder called 2019. The issue manifests itself by the user not being able to start the LOB app with an error relating to not being able to access the files that it needs on the server. There are many folders/files in this particular mapped drive including a folder that contains source and data files for a line of business app (LOB) that they use. Every so often one of those mapped drives (F:) appears very strange. Users' computer has 3 mapped network drives. Got a weird intermittent issue with one of our clients.