Hi Christian,
One other user had a problem with the invoice sync and we discovered a bug there. It is now fixed in the next version submitted. For right now, the best thing to do to temporarily fix this is to do a Backup on both the iPhone and the iPad (which as the correct data). Take the iPad backup and Restore it to the iPhone, so that the invoices are correct on both side. Sorry for the trouble.

Cheers,
Adam.