Go to "Library Setting" of the document set
Select the content type of interest
Go to "document set settings"
Under the section "Welcome Page Columns", select fields (columns) needed for the Document Set Properties Webpart
After this steps the values of the column are available on the welcome page
To prevent this in SharePoint Online I went Cog > Site Settings > Site libraries and lists > [Select doc library] > Change new button order and default content type > remove tick from content type leaving only doc set
This only allowed doc sets to be created at the library level and then only allowed content type documents to be created at the doc set level