Rest api carts/mine/set-payment-information and additional_data

When I follow the rest API docs and post some data to the carts/mine/set-payment-information endpoint e.g.

{
    "paymentMethod": {
        "method": "purchaseorder",
        "po_number": "123",
        "additional_data": [
            "this data"
        ]
    }
}

The method and po_number gets updated, but something happens in the additional_data field. How come?