When you initiate SLO to an IdP, it will log the user out of the local authentication session at the IdP and the IdP will in turn initiate SLO to the SPs with authentication sessions within the same browser session. The IdP has no knowledge of other IdPs.
If an SP has completed SSO with two IdPs, it would have to initiate SLO to both of these if the user is to be logged out from both IdPs and all associated SPs.
I couldn’t see any obvious issues in your code etc.
If there’s still an issue, please enable SAML trace at both the IdP and SP and send the generated log files as email attachments to support@componentspace.com mentioning your form post.
For SAML for ASP.NET:
Enabling SAML for ASP.NET Trace
For SAML for ASP.NET Core: