[BUG] Separate page section backgrounds reset by Add-PnPPageSection and Add-PnPPageWebPart

Issue

Section backgrounds are reset when updating a page with Add-PnPPageSection or Add-PnPPageWebPart and probably other commands.
Only the new section backgrounds are affected (i.e. not emphasis 0, 1, 2, 3 but rather the additional new ones that have gradiants images etc)

Expected behavior

Expecting Add-PnPPageSection, Add-PnPPageWebPart and other similar commands to not change separate sections on the page

Actual behavior

All section backgrounds that use the new backgrounds are reset on the page.

Before	After

Steps to reproduce behavior

1. Create a page on a site using the UI
2. Add a couple of sections.
3. Change the section backgrounds to use the new ones (gradients etc)
4. Publish the page
5. Run commands to update the page. For example

$site = "Your_site_url"
$pageName = "Page_name"
Connect-PnpOnline $site
$page = Get-PnPPage -Identity $pageName 
Add-PnPPageSection -Page $page  -Order "0" -SectionTemplate "OneColumn"

6. Reload the page in the browser. Note that all the new section background are gone

Note that the issue can also be reproduced with Add-PnPPageWebPart

What is the version of the Cmdlet module you are running?

2.5.0

Which operating system/environment are you running PnP PowerShell on?

• Windows
• Linux
• MacOS
• Azure Cloud Shell
• Azure Functions
• Other : please specify

Start with the Add-PnPPageSection and Add-PnPPageWebPart entry points and reproduce the issue using the commands and page setup described. Trace how updating a page serializes its sections; done means adding a section or web part preserves existing new section backgrounds, including gradients and images.