It is a by design behavior because the owner permission works for that resource group, not for the subscription.
If you want to grant create resource group permission to that account, You need to explicitly set permissions for it.
Grant the owner permission of this subscription to that account, in this way that account will have permission to create new resource group.
Note: If we grant owner permission of this subscription to that account, that account will get all permission of all resource group.
Hope this helps!!
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