I feel like a lot of the issues are also due to Azure making it vastly more difficult than necessary to configure OIDC/SAML and get the right information over to the SP.
I don’t doubt that, however the vendor doesn’t get a choice over the IDP and Azure isn’t even the worst! Last week we had to create a custom Metadata XML file for a customer because their IDP doesn’t accept self-signed certificates. For a certificate we send them. That we can validate on a call with them. And it’s completely undocumented in their IDP’s docs.
"Your issue is coming from a misconfigured SSO. We disabled SSO on your account, you can login with the standard password reset flow. You can reenable SSO once you have fixed the issue."
Azure AD or whatever they are calling is now isn't even fully SAML 2.0 compliant, so of course it generates support calls. Then the answer from MS support is "use this workaround" and now your implementation is locked into Microsoft's implementation.