I think we can consider this a launchpadlib bug. The WADL file describes the document type at the other end of task_target. It's accurate, but in this case there's a more detailed description of that document available. The link to this other, more detailed description is in the document's resource_type_link.
So launchpadlib should use a document's actual resource_type_link in preference to any resource_type it obtained earlier from another source. The document's resource_type_link, if present, will never be less accurate than the other source.
I think we can consider this a launchpadlib bug. The WADL file describes the document type at the other end of task_target. It's accurate, but in this case there's a more detailed description of that document available. The link to this other, more detailed description is in the document's resource_type_link.
So launchpadlib should use a document's actual resource_type_link in preference to any resource_type it obtained earlier from another source. The document's resource_type_link, if present, will never be less accurate than the other source.