I'm at a complete loss here. ArcGIS Enterprise 10.9.1.
I have existing webhooks via Portal that work with Power Automate just fine.
I have a very simple test survey and have created the ArcGIS Enterprise Connector as described here. The connector steps all validate correctly and the test webhook is created in the survey which I can see in Survey123 web designer.
When I add the custom connector to a new flow and add a simple step such as send an email and then save the flow, the new webhook is successfully registered with the Survey. When I attempt to test the flow it simply sits with a spinning icon which eventually times out.
I'm guessing since the connector steps all validated the answer to this is yes but is your ArcGIS Enterprise environment accessible over the internet?
If so after Survey123 sends the applyEdits request to the feature service it should also be sending a request to the payload URL of your webhook (which you should see on the Webhooks settings page in the Survey123 website). If you capture the web traffic while you submit a record are you seeing that request to the payload URL?