Well the idP performs SP initiated SAML SLO fine for users with standard Salesforce licenses who SSO directly into the Community.

I think Azure may just not support idP initiated SLO for applications that federate with SAML. I can't find any support for idP initiated SAML SLO in their docs.

The error message does make me suspicious of a general issue though (see attachment). It is odd we are getting a Sign In error on a logout request. It probably has to do with what you said about the endpoints being the same.

So right now I'm focusing on the SP initiated flow since Azure seems to have that working. We just need to figure out why Salesforce is not sending the LogoutRequest for users with Community licenses.