Try to add appservice config file for synapse to use

group_vars/matrix_servers

@@ -1339,6 +1339,11 @@ matrix_cactus_comments_enabled: false
 
 matrix_cactus_comments_container_image_self_build: "{{ matrix_architecture not in ['amd64', 'arm32', 'arm64'] }}"
 
+matrix_synapse_app_service_config_files: |
+  {{
+    (['{{ matrix_cactus_comments_app_service_config_file }}'] if matrix_cactus_comments_enabled else [])
+  }}
+
 # Normally, matrix-nginx-proxy is enabled and nginx can reach cactus-comments over the container network.
 # If matrix-nginx-proxy is not enabled, or you otherwise have a need for it, you can expose
 # cactus-comments web-server port.