Custom record field aggregate sum 'Test' shows in Field editor, but not in 'Data Preview' editor

Hi all,

I'm a bit new with custom record fields, so I'm probably doing something wrong.

I have header and detail records (with a one-to-many relationship connected with a detail reference to the header key), and that data flows throughout the system just fine. 

But I added a real-time aggregation / sum custom field (for totaling amounts in the detail record for presentation) and am having trouble getting data into the new fields.

When pressing the 'Test' button in the Edit Custom Record Field screen, I see data grouped and summed as needed which is great!

After saving the new custom field, the 'Data Preview' test button does NOT return any data in the new field, and that is also the case with an interface that references this.

I've done resync on both records.  The new custom field exists on the detail side of the relationship and when defining the field, I use the format: headerRec.detailRec.amountToBeSummed.

I've also tried coming at it from the header side (starting with related detailRec), but still have the same issue.  I'm testing this out as admin, so I can't think that it's a security thing.

Any ideas?

Here is good data test when considering only the field:

And here is data preview that cannot populate the new field: