istio JWT authentication for single service behind ingress gateway

I have 2 services running on AKS (v1.16.13) and deployed the following istio (v1.7.3) configuration. First one is a UI where I invoke the OIDC flow and get JWT token, second one is a backend service which should require a valid JWT token.

apiVersion: networking.istio.io/v1beta1
kind: Gateway
metadata:
  name: myapp-gateway
  namespace: "istio-system"
spec:
  selector:
    istio: ingressgateway
  servers:
  - hosts:
    - myapp.com
    port:
      name: http-myapp
      number: 80
      protocol: HTTP
    tls:
      httpsRedirect: true
  - hosts:
    - myapp.com
    port:
      name: https-myapp
      number: 443
      protocol: HTTPS
    tls:
      credentialName: myapp-credential
      mode: SIMPLE
---
apiVersion: networking.istio.io/v1beta1
kind: VirtualService
metadata:
  name: myapp
  namespace: myapp
spec:
  gateways:
  - istio-system/myapp-gateway
  hosts:
  - myapp.com
  http:
  - match:
    - uri:
        prefix: /ui
    route:
    - destination:
        host: myapp-ui.myapp.svc.cluster.local
        port:
          number: 4200
  - match:
    - uri:
        prefix: /backend/
    rewrite:
      uri: /
    route:
    - destination:
        host: myapp-service-backend.myapp.svc.cluster.local
        port:
          number: 8080
---
apiVersion: security.istio.io/v1beta1
kind: RequestAuthentication
metadata:
  name: myapp-jwt-backend
  namespace: myapp
spec:
  jwtRules:
  - issuer: https://oktapreview.com
  selector:
    matchLabels:
      app: myapp-service-backend

With that config I would expect to get 401 if I invoke myapp.com/backend but that's not the case. Request authentication doesn't kick in.

After some further research (https://discuss.istio.io/t/cannot-use-jwt-policy-with-an-externalname-virtualservice-target/2794/3), I found out that I can't apply RequestAuthentication on a VirtualService but only on a Gateway which is quite odd to me but ok. I've changed the RequestAuthentication to the following but still nothing has changed after invoking backend:

apiVersion: security.istio.io/v1beta1
kind: RequestAuthentication
metadata:
  name: myapp-jwt-backend
  namespace: istio-system
spec:
  jwtRules:
  - issuer: https://oktapreview.com
  selector:
    matchLabels:
      istio: myapp-gateway

Do you have any idea how can I setup istio for my use case? Assuming the RequestAuthentication would work on a gateway, do I need 2 gateway? 1 for UI and the second for backend? Seems like an overkill.