I have been working with Microsoft lately on an issue that I was experiencing with an Azure application gateway (appGW) deployment that require both internal and external interfaces handling traffic over HTTPS.
If you try to attach the same AppGW front end port to internal and external front end configuration, this would cause the appGW to misbehave. In my scenario I had a rule attached to external interface for handling incoming traffic but no rule attached on the internal interface and as a consequence the internal interface started processing traffic while the external interface was rejecting all connections (not even 502 error! would you believe!). Just to note that all my appGW deployments are scripted using PowerShell/JSON.
Microsoft managed to replicate this internally and issued a bug report, they are working on it as we speak but without ETA currently.
I had to drop my second listener (internal) in order to bring the appGW back to it’s expected behaviour!