HTTPRouteFilter's urlRewrite does not take effect under backendRefs

> The urlRewrite of HTTPRouteFilter does not work under backendRefs, but it works under rules. Is this as expected?
```
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: HTTPRouteFilter
metadata:
name: backend-1-credential-injection
namespace: envoy-gateway-system
spec:
urlRewrite:
hostname:
header: x-custom-host
type: Header
```
```
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: Backend
metadata:
name: backend-node3-fqdn-1
namespace: envoy-gateway-system
spec:
endpoints:
- fqdn:
hostname: application-1.hardybrown.com
port: 65201
```

-------not work config----
```
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
name: httprouter-to-node3
namespace: envoy-gateway-system
spec:
parentRefs:
- name: eg
hostnames:
- "envoy.hardybrown.com"
rules:
- backendRefs:
- group: gateway.envoyproxy.io
kind: Backend
name: backend-node3-fqdn-1
filters:
- type: ExtensionRef
extensionRef:
group: gateway.envoyproxy.io
kind: HTTPRouteFilter
name: backend-1-credential-injection
matches:
- path:
type: PathPrefix
value: /
```
-------work config----
```
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
name: httprouter-to-node3
namespace: envoy-gateway-system
spec:
parentRefs:
- name: eg
hostnames:
- "envoy.hardybrown.com"
rules:
- backendRefs:
- group: gateway.envoyproxy.io
kind: Backend
name: backend-node3-fqdn-1
matches:
- path:
type: PathPrefix
value: /
filters:
- type: ExtensionRef
extensionRef:
group: gateway.envoyproxy.io
kind: HTTPRouteFilter
name: backend-1-credential-injection
```