SAML with mod_auth_mellon - apache

Question

I've configured apache to make SAML auth for Grafana but the "X-WEBAUTH-USER" is not transfered to the header :
nc -l -p 9119
POST /grafana/ HTTP/1.1
Host: 127.0.0.1:9119
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:72.0) Gecko/20100101 Firefox/72.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Referer: https://samlidp.example.com/
Content-Type: application/x-www-form-urlencoded
Origin: https://samlidp.example.ch
DNT: 1
Cookie: mellon-cookie=cookietest
Upgrade-Insecure-Requests: 1
X-WEBAUTH-USER: (null)
Here is my config :
ServerName servername.com
ServerAdmin webmaster#servername.com
ServerAlias servername.com
DocumentRoot "/var/www/html"
# Logs and diagnotic
LogLevel debug
SSLEngine on
SSLProxyEngine On
SSLCertificateFile /etc/apache2/ssl/server.crt
SSLCertificateKeyFile /etc/apache2/ssl/server.key
#SSLv2 and v3 are bad
SSLProtocol all -SSLv2 -SSLv3
SSLCipherSuite HIGH:!SSLv2:!ADH:!aNULL:!eNULL:!NULL
ProxyPass / http://127.0.0.1:9119/
ProxyPassReverse / http://127.0.0.1:9119/
<Location />
Require valid-user
AuthType "Mellon"
MellonEnable "auth"
MellonDecoder "none"
MellonVariable "cookie"
MellonSecureCookie On
MellonUser "NAME_ID"
MellonSetEnv REMOTE_USER MELLON_NAME_ID
MellonSetEnv "REMOTE_MAIL" "email"
MellonEndpointPath "/endpoint"
MellonDefaultLoginPath "/"
MellonSessionLength 300
# Mellon requires a cert, regardless if it's actually being used.
MellonSPPrivateKeyFile /etc/apache2/mellon/urn_grafana.key
MellonSPCertFile /etc/apache2/mellon/urn_grafana.cert
MellonSPMetadataFile /etc/apache2/mellon/urn_grafana.xml
#MellonSPPrivateKeyFile /etc/apache2/mellon/urn_keycloak.key
#MellonSPCertFile /etc/apache2/mellon/urn_keycloak.cert
#MellonSPMetadataFile /etc/apache2/mellon/urn_keycloak.xml
# Make sure to copy your IdP metadata here
MellonIdPMetadataFile /etc/apache2/mellon/idp-persistent.xml
#MellonIdPMetadataFile /etc/apache2/mellon/idp-keycloak.xml
MellonSamlResponseDump On
MellonSessionDump On
RequestHeader set X-WEBAUTH-USER "%{REMOTE_USER}e"
RequestHeader set X-MAIL "%{REMOTE_MAIL}e"
</Location>
<Location /grafana/>
MellonEnable "off"
Order Deny,Allow
Allow from all
Satisfy Any
</Location>
Any ideas ?
I've tried this : Federate grafana with apache2 + mod_auth_mellon to have SSO with SAML but in that case the X-WEBAUTH-USER is not even in the header.