Thank you for your post.
I am unable to replicate the issue. This is what I did:
1. Using FileMaker Pro 13.0v3, I created three similar tables (Invoice, Items, Customers). I linked the Invoice and Items tables via an ID field). In the Invoice layout, I created a portal into Items, and I have the calculation fields Items = Count (Items::ID) and Subtotal = Sum (Items::Extended Price)
2. I entered a number of records, and the information displays as expected.
3. I created a new file (separation model), and added the Invoice and Items to the Relationships graph with the same linking of tables.
4. I created separate layouts for the tables, and made the layouts identical to the original including the portal.
5. I enter records, and the calculations are updated immediately. I do not need to commit the record.
Let me know what I'm doing differently than you so I can try to replicate the issue.