When I encountered save issues on android, I googled and stumbled on this topic https://github.com/cryptomator/cryptomator-android/issues/51
I am on Android 7.1 and latest Cryptomator app & still unable to save files to the open vault. The work around described in the link above creates more processes and leaves a copy of unencrypted file on another location in the device. Quite frankly, it is a lot of moving parts and when something breaks, harder to root cause. Having a simple solution to open vault -> make changes -> close vault… would be highly desirable.
If this is still open, I think the developers should describe the android app as read only. Especially when it is a paid app.