@kevin.chen Thank for the blog on kong + istio.
Did you try having 2 different versions of productpage micro-service and have a virtual service distributing the 90% traffic to productpage v1 and 10% traffic to productpage v2, the Kong would still distribute the traffic evenly to both version(As a kubernetes service would do). Since in your case productpage is hitting the review not kong.
I’m having issue hitting istio virtual service through kong. More details are described here: