@IainTollemache a really good question, and touches on a few Constellation concepts so i'm going to break it down. Short answer, this is expected, you should have a look at your user experience to avoid end user confusion.

1. Configuration:
   1. To confirm, your configuration is using a Data Reference, basically the ability to pick a "customer" from a list.
   2. This also supports the ability to display some details about them (see image below)
      1. 
   3. Your configuration also has case tabs the show details about the selected data reference from step 1
      1. 
   4. Your case has two steps in it. Step 1 where you select the data reference. Step 2, where you can update the data reference again.
2. Issue: The issue you raise, lies in the fact that the data reference is populated on step 1.
   1. Let's say at step 1 you select Bob and press submit.
   2. The data reference in your form view (editable area) is Bob and the customer information in case tabs is also Bob
   3. At step 2 you change the Customer from Bob to Frank
   4. Observed: The data inside the form area is Frank, the data inside the case tabs is Bob.
   5. Expected: The data in the case tab below should be Frank
3. Explanation:
   1. This has been Pega's interaction pattern through various UI Architectures.
      1. There are case tabs and details that show the current saved information on the case.
      2. A form that allows to add/edit details on the case, then submit or save to have this information saved on the case or the user could cancel to under all their current changes.
      3. This isn't new to Constellation, in fact, you probably remember we used to do all kinds of funky things in Perform and Review harnesses to avoid this issue, even going to some extremes with action sets to try to persist the data so it would update.
   2. In this case, your Customer selection (data reference) has not been saved or submitted, so it would be expected that the case tab below does not reflect this customer selected
   3. You can validate this by pressing cancel, you'll see your customer revert back to previous saved/submitted customer. Pega forms/assignments do no autosave, as we always give the user the ability to cancel their changes in case they have made a mistake.
   4. We can also consider this design pattern in context of other users looking at this case, and only seeing the case tabs OR running reporting (Insights). They should see the saved/submitted information, not information currently inflight being entered on a form, as this could be cancelled or abandoned at any time.
4. Solution: Don't despair, this is an easy fix in my mind.
   1. For me, we are just showing the "Full Details" tab, specifically the "Customer Information" too soon. By showing the unsaved selection in the form and saved selection in tabs we introduce the ability for end user confusion.
   2. I would recommend putting a visibility condition on this tab to only start to show this information after the customer selection has been made.
   3. Whilst it is still inflight, I would look to put any additional information you want to show on the customer in the Form View (mentioned in step 1.2) - this would allow the user to see the details of the selected customer (unsaved) so they can make an informed decision.

Hope that helps