Azure web app access restrictions to application gateway

I am using a V1 WAF application gateway (dynamic IP) in front of a web app. When i configure the web app access restrictions to Allow the application gateway i received a when i tryu to access the web app url (expected behavior). However, i also received a 403 when accessing throught the app gateway url (not expected !). i have try a rule to allow the app gateway ip and also a rule allowing the app gateway vnet (service endpoint) with both same result. Any idea why its not working ? Thx!