This is most probably there is mismatch of the port that application running in the DOCKER is not the same port exposed in the DOCKER configuration
Docker file EXPOSE <PORT> should be same as the port the application is started.
Istio Ingress gateway can bind to the container port but can't communicate the application.
I am not an expert (yet), but from what I understand, Istio allows you to check that someone (the principle of the call) is already authenticated/authorized.
But that actual authentication needs to have been done previously. You can use just about any Identity Provider to get the authentication done. Once done, the JWT from the authentication is added as a ...
Restarting Istio controlplane should fix the issue on hand (pods in istio-system, most likely pilot).
More importantly do not run Istio 1.2 its very outdated version with many bugs and security issues.