Using EnvoyFilters to patch routes overwrites filter_metadata

Networking
1

Hi

I’m trying to use EnvoyFilters to markup routes with filter metadata that a lua filter will later look for and use. However I’m noticing that after applying two envoyfilters that markup the same route the result is not what I was expecting.
For example, using the httpbin sample application and the filters below

apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: prop1-filter
spec:
  configPatches:
  - applyTo: HTTP_ROUTE
    match:
      context: SIDECAR_INBOUND
      routeConfiguration:
        portNumber: 8000
    patch:
      operation: MERGE
      value:
        metadata:
          filter_metadata:
            envoy.lua:
              prop1: "foo"
---
apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: prop2-filter
spec:
  configPatches:
  - applyTo: HTTP_ROUTE
    match:
      context: SIDECAR_INBOUND
      routeConfiguration:
        portNumber: 8000
    patch:
      operation: MERGE
      value:
        metadata:
          filter_metadata:
            envoy.lua:
              prop2: "bar"

I was expecting the resulting metadata to be

metadata:
  filter_metadata:
    envoy.lua:
      prop1: "foo"
      prop2: "bar"

However what I’m actually seeing is the second is overwriting the first resulting in

metadata:
  filter_metadata:
    envoy.lua:
      prop2: "bar"

I’ve tried this in Istio 1.3.4 and 1.4.2 with the same result. Is this correct behaiour?
Thanks in advance.