Record accessor in Labels does not work on all fields

I am trying to export a metric with labels created by kubernetes_metadata filter.
I'm using the following match:
```

@type parser
key_name log

@type regexp
expression upstream: \"http://(?.*?)/.*?\"

@type prometheus

name nginx_upstream_error
type counter
desc The total number of Nginx upstream timeouts.

error ${tag_parts[3]}
upstream $.upstream
origin-pod $.kubernetes.pod_name
origin-namespace $.kubernetes.namespace_name

```

Output in `/metrics` looks like this:
```
# TYPE nginx_upstream_error counter
# HELP nginx_upstream_error The total number of Nginx upstream timeouts.
nginx_upstream_error{error="timeout",upstream=":",origin-pod="",origin-namespace=""} 3.0
```

For some reason `$.kubernetes.pod_name` and `$.kubernetes.namespace_name` doesn't work and stay empty.
The source of this tag goes through the kubernetes metadata plugin:
```

@type kubernetes_metadata

```
I've validated that the object `kubernetes` with the specified fields is there (my whole logging infrastructure is based on it).
What is even stranger is that the `upstream` label works!
Seems like the record accessor does not work on fields came out from k8s enrichment but does work on fields came out of the previous parser.