GET /api/v1/tickets returns always assignee=null

GET /api/v1/tickets returns always assignee=null

I'm calling DESK API /api/v1/tickets with following parameter:
                'include' => 'assignee',
However, in result, 'assignee' is always null, even if the ticket is assigned to an agent.
This was working a few days ago, but does not work from today.

After some investigation - problem is only if i'm sending also the "fields" parameter.
Without "fields":
  "assignee" => array:5 [.....]
With "fields":
  "assignee" => null