Notebooks occasionally do not open in iOS 13 unless the device is restarted each time.
This issue sprouted up with the new iOS 13 & even until the last iOS version, there were still certain issues with the Apple API that were actually not under our control. There were reports on the internet of other apps having similar issues after the introduction of iOS13. As long as the iCloud sync feature is active, it is the Cloud that is responsible entirely for opening the notebooks in the app as well. We don’t have much control over such iCloud related things.
We wrote to Apple on this & they have fixed this issue with their latest OS update i.e. v13.3. Kindly update your device to iOS v13.3, update the app to the latest version on the App Store & restart your device once by pressing the power button for a few seconds to fix the issue. If the issue persists, please report it to Support via Gear icon->Support->Email Support option within the app.