Hi DaisyITGirl,
Thanks for your video in PM link.
I watched video and your post several times. I noticed that you have 2 scenarios. I test it at my end, I found if I create a Document Sets content type use the default Document Sets content type, the URL will all be Forms/Test%20Document%20Set/docsethomepage.aspx? and I can see the notification in page.
Per my test and research, when you click Edit Page to edit this content type welcome page. This change is for all this content type, it’s by design in SharePoint Online. In your Set Content Settings, you have many different Document Set types, they look like same. But if you Add different content type in Document library, when you edit different welcome page, they won’t update together. (Because of the different content type.)
We suggest you check the problematic content type, to see if they are the same kind of Document Set content type.
To make a explanation:
When i edit welcome page of Test Document Set content type, the welcome page of 1111 content type won't change.
Regards,
Ivy