Please check whether the identity provider is returning the relay state along with the SAML response.
If you take a look at the network traffic, the HTTP Post to your /eIDAS/AssertionConsumerService endpoint should include RelayState post data set to the URL you specified when calling _samlServiceProvider.InitiateSsoAsync.
If there’s still an issue, please enable SAML trace and send the generated log file as an email attachment to support@componentspace.com mentioning your forum post.