Synapse doesn't log out from keycloak when logout from server

[kachi-app]

Description

When I click on Logout button from my web apps, the sessions got cleared from synapse server, however not from Idp ( Keycloak ). I checked in to keycloak, and the sessions still exists in the client realms.
What I expected: When I logout from Synapse, Sessions in Keycloak got cleared / removed.

Steps to reproduce

• Install and setup Keycloak version 26.0.7
• Install Synapse Version 1.122.0
• Implement the logout button

Homeserver

GKE Internal

Synapse Version

{"server_version":"1.122.0"}

Installation Method

Docker (matrixdotorg/synapse)

Database

PostgreSQL 15.4.0

Workers

Single process

Platform

Run in GKE, using ananace helm charts https://artifacthub.io/packages/helm/ananace-charts/matrix-synapse

Configuration

      oidc_providers:
        - idp_id: keycloak
          idp_name: "chi-space"
          issuer: "https://example.com/realms/synapse"
          client_id: "synapse"
          client_secret: "xxxx"
          scopes: [ "openid", "profile" ]
          user_mapping_provider:
            config:
              localpart_template: "{{ user.preferred_username }}"
              display_name_template: "{{ user.name }}"
          backchannel_logout_enabled: true # Optional
          backchannel_logout_ignore_sub: true

Relevant log output

{"log":"10.164.0.55 - 8008 - {@testinguserfe:example.com} Processed request: 0.139sec/0.003sec (0.010sec, 0.003sec) (0.014sec/0.071sec/9) 2B 200 \"POST /_matrix/client/v3/logout HTTP/1.1\" \"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.6778.33 Safari/537.36\" [0 dbevts]","namespace":"synapse.access.http.8008","level":"INFO","time":1737628567.36,"request":"POST-240","ip_address":"10.164.0.55","site_tag":"8008","requester":"@testinguserfe:example.com","authenticated_entity":"@testinguserfe:example.com","method":"POST","url":"/_matrix/client/v3/logout","protocol":"HTTP/1.1","user_agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.6778.33 Safari/537.36","server_name":"example.com"}


In the keycloak with Log Level Debug, there are no logs indicate that `logout` was called

Anything else that would be useful to know?

Related Issues:
#14783

As I don't know is the collaborator / maintainer will notice the issues, So i created a new issues here.