I have some api deployed to azure webapp. In front of this app there is a Application gateway.

When I access the app through .azurewebsites.net url it works fine, but when I try to hit the same path through a custom domain of the app gw - it fails.

In application gateway logs in AzureDiagnostics part of it I see it fails with 499 error.

Application gateway has a number of multi-site listeners configured, is of Standard V2 tier (and therefore does not have any WAF applied) and manually scaled to 2 instances.

What is rejecting those calls? How can I overcome this issue?