why "Site Page" content type is missing the options under its Settings section, insides our root site collection

Question

I am facing this issue inside our sharepoint online root site collection:- when i tried to add some custom properties to our modern site pages, by going to Site Setting >> Site Content Types >> i click on the built-in "Site Page" content type. Where i thought i will have the options to add some custom columns to our "Site Page" content type, but i found that the content type does not have any options under the Settings + there is not option to add columns to the content type, as follow:-

while if i create a new site collection let say @ https://ourcompanyname.sharepoint.com/sites/siteA , then the "Site Page" Content type will have "Advance Setting" link under the Setting + will have options to add columns to the content type ("Add from existing site columns" link), so i were able to add cusotm properties to it, as follow:-

so is my issue only happen at the root site collection? and how i can fix this?

Answer 1

The reason is the content type "Site Page" at "/" is sealed. I don't suggest you modify it, if you still want to add custom columns to this content type, try to use PnP PowerShell to achieve it.

$credential = Get-Credential  
Connect-PnPOnline -Url https://xxx.sharepoint.com/ -Credential $credential
$ctype = Get-PnpContentType "Site Page"
$ctype.Sealed = $false
$ctype.Update($true)

Answer 2

It looks like the update that "fixed" this issue (ie unsealed the site page content type) only works on sites created after the update was applied to your tenant. Msft seems totally unaware of this and oblivious to the problems and confusion it causes for the majority of their customers with pre-update sites. Run the powershell unseal script above in an affected site and it should fix the problem. I would also contact Msft via service request and ask that they confirm this is the cause and come up with a supported, tenant wide solution.