Shipment API won't append comment to email

[JeroenVanLeusden]

Duplicate of #4320 but since no activity there I created a new one with more detailed information.

Preconditions

1. Magento v2.1.9
2. PHP v7.0.14
3. MySQL v5.6.33

Steps to reproduce

1. Enable async email sending.
2. Place an order via the website.
3. Create a shipment using the API with "appendComment": true.

Expected result

1. Shipment email is being send with comment appended.

Actual result

1. Shipment email is being send without comment appended.

API call used

<domain>/rest/all/V1/order/1/ship

{
  "items": [{
    "order_item_id": 1,
    "qty": 1
  }],
  "notify": true,
  "appendComment": true,
  "comment": {
    "comment": "You order will be delivered between 12.00 and 14.00.",
    "is_visible_on_front": 1
  }
}

Workaround

This workaround will use the first comment found with getIsCustomerNotified which is enough for our use case.

Create an after plugin for the \Magento\Sales\Model\Order\ShipmentDocumentFactory::create

public function afterCreate(ShipmentDocumentFactory $subject, ShipmentInterface $shipment)
    {
        foreach ($shipment->getComments() as $comment) {
            /** @var ShipmentCommentInterface $comment */

            if ($comment->getIsCustomerNotified()) {
                $shipment->setCustomerNote($comment->getComment());
                $shipment->setCustomerNoteNotify(true);

                break;
            }
        }

        return $shipment;
    }

[magento-engcom-team]

@JeroenVanLeusden, thank you for your report.
We've created internal ticket(s) MAGETWO-80813 to track progress on the issue.

[JeroenVanLeusden]

I'm working on it #SQUASHTOBERFEST

@okobchenko How you want to see this fixed? Maybe I can give it a go.

I can implement the solution I have right now and only add the first found comment in the email or combine all comments into one message.