I don’t see any obvious issues with the code.
We use a saml-session cookie in support of the SAML protocol. It’s possible this cookie wasn’t sent with the logout request.
Please enable SAML trace and send the generated log file as an email attachment to support@componentspace.com mentioning your forum post.
Enabling SAML for ASP.NET Core Trace
Please ensure the log includes both the successful SSO and failing SLO.