We've been running Enterprise 12.1 since 19 June. We use the ESRI hosted survey123.arcgis.com website to manage report templates, webhooks, ect. for our published Survey123 surveys. We obiv. access the website by adding our Enterprise portalUrl as a query parameter, like: https://survey123.arcgis.com?portalUrl=gisportal.ourdomain.com/portal
No issues there, I see all the published surveys I own and can browse all the survey records and their associated photos, no issues. Our Enterprise 12.1 site is hosted in Azure, behind a Application Gateway, that passes all :443 traffic on the public IP to a backend webadaptor machine, running /portal and /server instances.
A week after upgrading to Enterprise 12.1, I used the survey123.arcgis.com website to update a report template, that was done on 26 June, no issues.
The Problem:
21 July, I noticed everything related to the Report API doesn't work on survey123.arcgis.com. All of the operations listed on that help doc return Error: 404 service not found. Browser dev tools show the Report API requests to those operations are being sent to gisportal.ourdomain.com/server/ They should be going to survey123.arcgis.com, correct?
Here's the request that's sent when you try and create a sample template on survey123.arcgis.com.

The payload of the request seems to be OK, the featureLayerUrl is correct and accessible in a browser

We have a webhook that triggers a PowerAutomate flow, and the HTTP flow action is correctly using the https://survey123.arcgis.com/api/featureReport/createReport Request URL and that is working without any issues.
This seems to be an issue with the survey123.arcgis.com website.
Anyone else having Report API issues ?