gRPC server can't respond after 30~40min

I use GKE with Istio on GCP and I implement a simple gRPC hello server, create Gateway and VirtualService.
At that time, all works good. gRPC client on my local PC can call hello api.

After some tests, I find I don’t set ‘istio-injection=enabled’ to my namespace.
So I set ‘istio-injection=enabled’, and uninstall / install hello server.

gRPC communications between client and server works well at first. After 30~40 mins., client gets gRPC deadline errors even though no requests previous 30~40 mins.

My questions are

  1. I think some timeout issues in istio. Where can I start to debug ?

  2. When I missed “istio-injection=enabled”, there are no problems in request routing using Gateway and VirtualService. Is it all right ?