The MuseScore app (with the integrated Songbook functionality) fails to open MuseScore files (e.g. on the file system or as e-mail attachments). The MuseScore app is simply not suggested as an app to open a .mscz file with.
Any other users having issues with this?
The good old MuseScore Songbook app (not available anymore for new users) doesn't have any problems with that on my Android devices.