PowerDNS API changetype REPLACE does not replace a RRset

Question

I'm making the following API call to PowerDNS, expecting to replace the old record value oldhostname.example.net with the new value newhostname.example.net.

PATCH /api/v1/servers/localhost/zones/example.org
X-API-Key: secret
Content-Type: application/json
{
  "rrsets": [
    {
      "name": "test.example.org.",
      "type": "CNAME",
      "ttl": 3600,
      "changetype": "REPLACE",
      "records": [
        {
          "content": "newhostname.example.net.",
          "disabled": false
        }
      ]
    }
  ]
}

The result is 400 error from the API with the messsage "The CNAME record you are trying to create already exists."

I know the record test.example.org already exists, my intent was to replace it with the new content.

Making two separate calls, one to delete and another to add, works. But I want to avoid any momentary NXDOMAIN responses to queries between the 2 API calls which could be negative cached on other DNS forwarders in the environment and cause a longer than necessary outage.

If "changetype": "REPLACE" does not actually replace the record, then what is the correct way to update a record in this API?