'tmpFromAccountId' History Item Field

Hi, we’re developing a plugin that processes issue history. When assignee changes history item look like this.

{
  "field": "assignee",
  "fieldtype": "jira",
  "fieldId": "assignee",
  "from": "ahmet",
  "fromString": "Ahmet",
  "to": "emre",
  "toString": "Emre",
  "tmpFromAccountId": null,
  "tmpToAccountId": "5ce*************44c"
}

The problems is that tmpFromAccountId field is always null. Why is it null and how can we get this fields value ?