Certain apps occasionally won't unhide once hidden
I'm getting this situation as well. The ways I've found to make them show again after switching to each app is either


Selecting Window > Zoom and then resizing it after it appears, filling the screen.
Selecting View > Enter full screen (or ^-command-F) then exiting full screen.


I haven't found a way to prevent it from happening. It doesn't happen to every app or happen every time I hide apps and close my laptop.