AD FS supports both HTTP-Redirect and HTTP-POST bindings for SAML Logout requests.

Azure AD does not support the POST binding. This means that not all applications can be migrated successfully from AD FS to Azure AD.

Please allow the logout requests to be POSTed to Enterprise Apps that only support this binding, and take notice of the supported bindings when uploading a service providers federation metadata document (which states what the service provider supports) when setting up an Enterprise App.