I stumbled across the same problem while testing the SAML integration and I finally managed to get it to work. With logs enabled, check the content of @attributes, those are the actual keys you need for the mapping and not the friendly name. They correspond to the Name attribute from the XML data. For my case the mapping inside app.yml became:
DISCOURSE_SAML_REQUEST_ATTRIBUTES: "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/emailaddress"
DISCOURSE_SAML_ATTRIBUTE_STATEMENTS: "email:http://schemas.xmlsoap.org/ws/2005/05/identity/claims/emailaddress"
It is also important to rebuild after changes to the config. After doing so, it properly mapped the email value from my SAML response.