Am in the process of migrating our current SharePoint Online Classic sites and pages to the new modern layouts. In some cases we are taking the opportunity to flatten our structure and reorganize data where appropriate. Given this work I have created a new top level site to which I created site columns and then site content types. Some of these content types are based on the Document Set Content Type. I am able to create new content types, add site columns, and set other items that should apply to any instance or use of the content type. What I am unable to do is modify the Welcome Page for a Document Set Content Type at the top site level. I can access the Welcome Page at the site level, but all page menu items are greyed out. If I apply a site content type to a specific library, I CAN edit the Welcome Page.
I have spent nearly 3 days discussing this issue with a very nice, but ultimately ineffective, representative at Microsoft. Does anyone know of a reason why the Welcome Page of a site level content type can not be edited, but the Welcome Page of a content type within a given library can be?