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Maybe Google has a bug and doesn't parse the license link if it's
a href=... rel="license" instead of
a rel="license" href=...?? Putting the
rel in a more common position is a trivial change worth trying (even if it's silly, it doesn't cost anything).
Or Google gets confused by the "license" linking a non-standard URL? I'm not sure what's the point in machine-readability of a URL that has no meaning for a machine, indeed. I don't remember what's the standard for this right now.