Sorry for the delayed reply. The member of the Discourse team who I think could help with this is away this week. Let us know if you are able to get the logout issue sorted out before then.
Here are a few links to topics that might provide some help:
- Discourse-saml: SLO does not cancel session
- Discourse SSO module for SimpleSAMLphp
- Discourse SSO via a SimpleSAMLphp authentication module (IdP)
It might be worth trying to contact the author of the SimpleSAML.php module to see if they have any ideas about how to fix the issue.