Created attachment 202614 [details]
headerProperties.odt: example file

Changes to the header properties are only (visibly) made to the main header (Master) format - and not to the Left pages, or to the "First Header" page. The change only becomes fully realized after a save and reload. (Switching to web view and back is not enough, so it is not simply a layout or invalidation issue.)

Steps to reproduce:
1. Open headerProperties.odt: 5 pages with separate first, even, and odd headers
2. Click on the "first" header - Choose "Border and Background" from the context menu.
3. Change the background to some colour, gradient, or whatever

Expected: all page headers will get that background change
Actual: only the odd/right pages (3,5) are affected - not even page 1 which was the page we thought we were modifying.

4. Save and reload

Expected and Actual: all page headers now have that background change.

While headers can have different CONTENTS for first and left pages, there can only be one format (background, header text margins, spacing, height, etc.). Any of these other Format - Page Style - Header properties can be substituted for the background colour in the steps to reproduce.

Ultimately, there can be only one (shared) header property because styles.xml only defines it once as:
<style:page-layout style:name="Mpm1">
  <style:header-style>
    <style:header-footer-properties fo:background-color="gold" ... />
  </style:header-style>
</style:page-layout>