(http://curl.haxx.se/bug/view.cgi?id=2813123) and an a patch that fixes the
  problem:

  Url A is accessed using auth. Url A redirects to Url B (on a different
  server0. Url B reuses a persistent connection. Url B has auth, even though
  it's on a different server.

  Note: if Url B does not reuse a persistent connection, auth is not sent.