I was informed that after MMJ changed the file format for the exports, Premium needed to be updated to allow the tracks to be imported once again.
It's been a month or more and I'm still waiting.
I was attracted to MMJ and MMM precisely for this ability to export projects from the android app and import them into MMM, but haven't been able to do so now for some time. I have a half dozen projects lined up that I cannot finish in MMJ, and I'm getting fed up. This was a major selling point of MMM and it's hardly been a year before it's been broken. It's still being sold as compatible, too. I'm not a happy customer :-(