Multi-issue custom field update over REST

[Liz Frost]

I'm trying to update a custom field on a lot of issues, and I've found the `/issues/properties/multi` experimental endpoint. However, I cannot get my requests to work with custom fields.

I'm making requests like this:

{
 "issues": [
    {
      "issueID": 247406,
       "properties": {
         "customfield_12398": [
            {
              "value": "builder",
               "id": 12447
            }
         ]
       }
    }
  ]
}

I've tried several variations on this, including:

{
 "issues": [
    {
      "issueID": 247406,
       "properties": {
         "customfield_12398": [12447]
         ]
       }
    }
  ]
}  

I've tried several variations on this, including:

{
 "issues": [
    {
      "issueID": 247406,
       "properties": {
         "customfield_12398": ["builder"]
         ]
       }
    }
  ]
}  

All these are happily accepted by the endpoint, and it reports no errors, but there are no changes to the field.

I know this endpoint is experimental, but I'm hoping to avoid sending several thousand `/issue/:id/edit` requests.