Availability Group Read Only Routing Not Working for Azure App Service that connects over Hybrid Connection
Microsoft Support engineer replied below on my case:


  I (Support engineer) have reached my technical leads to have an urgent update on this and I was able to get it.
  Unfortunately, the Product Group confirms my initial suspicions about the setup you want to achieve not being possible due to Hybrid Connections limitations therefore not supporting this. It’s a by-design behavior and currently Hybrid Connections does not support this scenario. The Product Group engineers confirmed me that this feature is being considered however they have no ETA to share.


Hybrid Connections do not support AG read-only routing - as simple as that

I have created feature request on Azure website, please login and up-vote it if you can 
https://feedback.azure.com/forums/169385-web-apps/suggestions/38917648-enable-hybrid-connections-to-work-with-read-only-r