Main Menu url being re-written incorrectly

Thanks for that pointer - though reading the docs it makes me think this is a bug:

From Menus | Hugo

  • pageRef
    (string) The logical path of the target page, relative to the content directory. Omit language code and file extension. Required for internal links.

  • url
    (string) Required for external links.

I’m using url, as this should be treated as an external link.

What I’m not clear on is if this rewrite issue arises in Hugo or in a theme template