When configuring SAML one of the steps is to configure the identity provider with the service provider identity URL or URI, normally this would be done using the SP URL or some other unique ID however MM has coded theirs to use the Identity Provider request URL. This is incorrect as if every SP did this the IDP would have no way of telling them apart.
This can be seen here in step 8 https://docs.mattermost.com/deployment/sso-saml-adfs.html this link refers to the “relying party trust identifier” and links it to begin synonymous with the “identity provider issuer url” but these are two very different things. It is actually synonymous with the service provider identityas service provider and relying party are interchangeable references
This value should be a unique SP value not the value of the IDP. e.g. https://mattermost.example.com/login/sso/saml