It’s recognized just fine for the production url, but for instances behind a reverse proxy, or with a separate management node, having an internal url for the test webhook is useless.
Is there another undocument env variable that can be used for this?
Maybe you could tell us a bit more to provide context, because this issue is not far spread and there must be something in your environment or configuration that makes it ignored. Tell us about the installation method (docker, npm), architecture of your environment (how is the instance exposed) and share your configuration, specifically for env vars.