Round robin load balancing issue when using mtls port 15443 for cross clusters communication

Currently we deploy our service mesh on k8s clusters with multi primary multi network topology. It works fine except for the load balancing issue. when we access the service in another k8s clusters, the traffic is not evenly distributed to the service pods. We have raised the issue to envoy but not sure whether it is related to our istio config setting. we didn’t set any destinationrule for our service, we think it is using round robin load balancing by default.

May i know how to set the config to make the traffic round robin distributed across the cluster ? thanks.