Remote REST API getTasksAssignedAsPotentialOwner() doesn't return task

Issue

In Remote REST API, customer try to get un-assigned tasks by the following Remote REST API.

List<TaskSummary> tasks = taskService.getTasksAssignedAsPotentialOwner("bpmsuser", "ja-JP");

But this doesn't return task even though there is a task whose potential owner matches specified user.