Hi Philip,
Sorry your first post was missed, and subsequent reply, only just saw this now. As I'm sure you know, we get a lot of new posts and replies every day on GeoNet, so sometimes one or two slip through. However, if the issue is high priority and needs urgent attention, the best avenue is to log a support ticket with Esri Support.
What version of the field app and web app were you using on the devices? Also what make/model and OS version did you see this happen on?
After you added the webhook in Flow and/or Integromat, did you re-download the survey on the device? This is important otherwise the webhook information will be missing.
Have you tried using Charles or Fiddler to analyse the network traffic when you submit, do you see the webhook payload being sent every time, along with the feature payload to ArcGIS? Also did you review the app logs to see if the webhook is actually being sent. It would be good to know where the webhook is going a miss, at the app level or not being received by webhook provider?
We have other customers with surveys that use webhooks that send hundreds and in some cases thousands of surveys a day from the web app and field app, and for each one a webhook also gets sent. Once customer is sending 3,000 a day, all without failure and every feature and webhook being submitted successfully.
It is a shame you decided to abandon Survey123 so quickly, as I am sure we can get the bottom of the issue you are experiencing with webhooks.
If you want to troubleshoot this more, please let me know.
Phil.