Running configurable-http-proxy externally

Hello,
I run a JupyterHub instance in docker. I want to setup configurable-http-proxy externally (using docker image jupyterhub/configurable-http-proxy:4.2.1) mainly to allow users to get their notebooksalive in case of JupyterHub restart (e.g. for software maintenance)

Things sound quite ok, but as a test, if I stop JupyterHub for 3 or 4 minutes , JupyterLab (version 2.1.0) prompts a dialog box "Server Not Running "
Whay this ? my notebok is still running (checked by running a python for ever loop)
(Using JupyterHub 2.1.0 the kernel is not lost)
Why I have to deal with this "Server Not Running " dialog box ?
I thouhgt that once a notebook was spawned, all user’s connections goes thru proxy
Is that correct or not ?
Regards,
Bernard