Hi Antonio,
thank you for your reply.
I was able to resolve the issue by bypassing Nginx and handling everything directly through Apache. We set up a new server running on Apache, and we are no longer experiencing the problem of headers being lost.
I believe the issue was related to the Nginx → Apache → Shibboleth chain, where headers were likely being dropped along the way.
Thanks again for your support.
Best regards.
Gacalin Lufi