...
Use of a stateless IdP login handler does not make a deployment completely stateless. The login context is still persisted using server-side state which must be present through the process beginning with the receipt of an AuthnRequest and ending with the issuance of a response. There must be either short session stickiness to associate a client with a single node for ~5 minutes, or however long the login process may take(~5 minutes), or replication of the IdP session object itself using another clustering mechanism.
...