Hi @Roger Roger
You could try this cmd Remove-SPOHubSiteAssociation https://contoso.sharepoint.com/sites/Research
, which removes the research site from the marketing hub site.
For more information, you could refer to:
https://learn.microsoft.com/en-us/powershell/module/sharepoint-online/remove-spohubsiteassociation?view=sharepoint-ps
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