Completing web form on site does not trigger registered webhook

I'd like to take action on an app for a client so that when a user submits a web form, I can then process that data via a webhook. I have authorized my Adobe Sign App in my developer account to have access to the user account that owns the web forms. I also have, among other scopes, the agreement_read:self scope set. I can call the agreements api via my app and see all the agreements created by users signing these web forms without any issue. However, the webhook I have registered never gets triggered by any actions of a user submitting the web form. Just to test, I enabled a whole range of events. I would expect that AGREEMENT_ALL would trigger a webhook response when a user fills out a web form, but it doesn't seem to.

Why is my webhook not firing under these conditions? It doesn't seem like it would be an access issue as I'm able to query the /agreements endpoint and see all the agreements resulting from the web forms.