Using relative URLs for content types across site collections is not supported

In Microsoft SharePoint Online, when you specify the Document Template for a site content type by using a URL of an existing document template, make sure that you use the URL of an existing site in the same site collection. A relative URL to a content type is not supported in the content type syndication scenario.

Specifically, under Site Content Type > Advanced Settings > Document Template, the URL (if any) has to be a URL of an existing content type on the same site collection.

Enter the URL for Document Template in Site Content Type: Advanced Settings page

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