Default site mapping does not work


According to the documentation, the default site mapping is entered like this:
To set the default login page (the page that will be used if no IP match) use default as key value:

This is not working, something is appending an additional "_login/" to the URL and external users are being redirected to https://sharepoint.website.com/_login/_login/default.aspx. I have tried various combinations of mappings "default.aspx, /default.aspx, ./default.aspx) and then external users just get a page can not be displayed error.