REST API does not provide tag_names field for controllers

[Louis Laporte]

When a region controller is tagged following this steps:

$ maas my-maas region-controllers read | jq -rc .[].system_id
g8tmfb

$ maas my-maas tags create name=deployer
Success.
Machine-readable output follows:
{
    "kernel_opts": "",
    "definition": "",
    "name": "deployer",
    "comment": "",
    "resource_uri": "/MAAS/api/2.0/tags/deployer/"
}

$ maas my-maas tag update-nodes deployer add=g8tmfb
Success.
Machine-readable output follows:
{
    "added": 1,
    "removed": 0
}

$ maas my-maas region-controller read g8tmfb | jq .tag_names
null

$ maas my-maas node read g8tmfb | jq .tag_names
null

I was expecting to retrieve the tag in the region controller definition, same as machine does.
In the web UI region controller tag is present.

Example for compute node:
$ maas my-maas machine read pm6pxt | jq .tag_names
["compute"]

The only way the tag_names of the region controller can be retrieved is to iterate over all tags and check if the region controller ID is present.

$ maas my-maas tag nodes deployer | jq -rc .[].system_id
g8tmfb