Other TurboTax Business users have had the same experience with partners (1065) and shareholders (1120-S).
There does not appear to be one particular cause, although a few have been mentioned with one of them being an inadvertant deletion of a partner or shareholder on the previous year's return (i.e., it was indicated on the prior year return that the partner or shareholder left the partnership/LLC or S corporation). Another issue was, somehow, a K-1 was marked as final.
One solution was to correct the prior year return and re-transfer while the other was simply starting the new return from scratch. The issue was escalated to the developers, but there has been no further action taken to date.