If you are using a legacy template (I tested with eZ Flow) and have an eZ Publish kernel error (I tested with Kernel Error 20, Module Not Found), the error template will fall back to eZ Demo not to the standard design.

The parameters.yml file has a key that seems to be responsible for this fallback to ezDemo but, since the legacy templates are not bundles, I can't see how to configure this stay in the legacy.

 
ezpublish_legacy.view.default_layout: eZDemoBundle::pagelayout.html.twig