When an app such as PDF Notes S or PDF Notes C is updated,
in the [My iPad] or [My iPhone] location,
you may experience problems with disappearing app folders
such as [PDF Notes S] or [PDF Notes C].
After screen lock of iPad or iPhone,
(not using FaceID nor TouchID)
enter your passcode to unlock the screen,
you will see the folders that you just didn't see before.
I hope Apple will fix this problem asap.