SSO Integration with Shibboleth IDP v4

I’m trying to integrate Bitwarden SSO with my personal installation of Shibboleth Identity Provider. Login authentication succeeds and my browser is redirected to the SSO ACS URL which gives me the error “The signature verified correctly with the key contained in the signature, but that key is not trusted.”

To be sure the X509 Public Certificate placed in the Bitwarden’s SSO configuration is a self-signed certificate, but given that certificate was provided in the IDP metadata file I don’t know why it wouldn’t be trusted.