Mirrored request with host rewrite has wrong Host header

Description: I'm currently trying to use request_mirror_policy in a route that is also using auto_host_rewrite. The problem are the host/:authority headers in the mirrored request is giving me weird results...

Repro steps: I created a simplified version of my setup in this gist including a full log of execution

docker-compose up --build

curl localhost:5000/any/host_rewrite

Mock server logs 2 requests

# The actual request
mockserver_1    | 	{
mockserver_1    | 	  "method" : "GET",
mockserver_1    | 	  "path" : "/any/host_rewrite",
mockserver_1    | 	  "headers" : [ {
mockserver_1    | 	    "name" : "host",
mockserver_1    | 	    "values" : [ "api" ] # successfully rewritten
mockserver_1    | 	  }, {
....

# The mirror request
mockserver_1    | 	{
mockserver_1    | 	  "method" : "GET",
mockserver_1    | 	  "path" : "/any/host_rewrite",
mockserver_1    | 	  "headers" : [ {
mockserver_1    | 	    "name" : "host",
mockserver_1    | 	    "values" : [ "localhost-shadow:5000" ] # Why localhost?
mockserver_1    | 	  }, {
....

Calling again...

curl localhost:5000/any/host_rewrite

Mock server logs 2 requests

# The actual request
mockserver_1    | 	{
mockserver_1    | 	  "method" : "GET",
mockserver_1    | 	  "path" : "/any/host_rewrite",
mockserver_1    | 	  "headers" : [ {
mockserver_1    | 	    "name" : "host",
mockserver_1    | 	    "values" : [ "api" ] # successfully rewritten
mockserver_1    | 	  }, {

# The mirrored request
mockserver_1    | 	{
mockserver_1    | 	  "method" : "GET",
mockserver_1    | 	  "path" : "/any/host_rewrite",
mockserver_1    | 	  "headers" : [ {
mockserver_1    | 	    "name" : "host",
mockserver_1    | 	    "values" : [ "api-shadow" ] # successfully rewritten

From now on it's always successfully rewritten, until I restart the containers and the same thing happen. Maybe this is some caching problem in the host resolution?

Tested versions envoyproxy/envoy:v1.10.0 envoyproxy/envoy-dev:daeb9850dbcd78da21a2e8ff9854564830cefb19 # 1.13.0-dev

Additional question As you can see in my envoy.yaml I want to mirror the request to another cluster with another domain (I'm migrating one feature from a monolith to a microservice).

So in my case the real request is going to api, and the mirror request is going to another cluster called api-mirror. In this case, shouldn't the request mirror get the host=api-mirror-shadow?

The documentation is not really clear (at least to me) which cluster is going to be used when rewriting, the mirror cluster or the original one?

Thanks for the help!