If you see the RFC docs https://tools.ietf.org/html/rfc7519#section-4.1 the iss, sub, aud fields are marked OPTIONAL. But if you test out the Istio configs without iss, sub and aud it does not seem to work.

You can see it in this Istio doc as well the issuer field is required https://istio.io/latest/docs/reference/config/security/jwt/#JWTRule