Dropbox causes temp hiding of client list
Using Dropbox was recently discovered in version 8.9 of iOS to sometimes hide the client list when you push a PDF document to Dropbox. The clients are not erased as it appears, however, they are hidden, and you must force quit and restart the application in order to see the client list again. This will be addressed in a future update.
Video link: https://youtu.be/nJHgW_ScWc4