Version: 15.0.2

Steps to reproduce the issues:
I was teaching 3 students on 15.0.2 (all on Windows) how to create a new document. We set up a 12-page file and specified 3 columns and changed the top margin in the New Document dialog box. With the preview checkbox on, body page 1 did not reflect the changes. We could see the bleed and slug lines appearing when defined. We could also see the A in the corner of the the body page 1 icon in the Pages panel, indicating that A-Master was assigned.

Expected result
When you define a new document with Preview enabled, you should be able to see the margins and columns as defined on the first page, behind the dialog box before (and after) you click on OK.

Actual result:
After clicking on OK, we discovered that the margins and columns were defined and visible on the A-Master and body pages 2-12. We had to manually re-apply the A-Master to Body page 1. This happened to all three students.

It is working as expected on my Mac 10.15.3.