Backorder customer notification is not shown in Magento 2.2.0 / 2.1.9 on One page checkout

[KarlComSe]

Preconditions

Magento 2.2.0
PHP7
Multi-site setup
Magento Luma theme.

Steps to reproduce

0. Go into stores > configuration > invetory : Set Display out of stock products: Yes, select Manage Stock: Yes. Set Backorders; Allow Qty below 0 and notify customer.
1. Create new simple product.
2. Set stock status, in Stock.
3. Set quantity below 0. In my case, this was done by customer buying a unit when it was at 0.
4. Select advanced inventory, check so that the Manage Stock is Yes, qty below 0 and for Backorders: Allow Qty Below 0 and Notify Customer.
5. Save product.
6. Go to storefront and add the product to cart.
7. Select Proceed to Checkout, in the minicart.

Expected result

1. Show message about "Backorder".

Actual result

1. No message is shown.

There is a notification only shown when one go to "View and Edit cart". Most customer jumps this step.
This is also reported here, but according to guidelines, was requested to be a new topic: #5538 (#5538)

[magento-engcom-team]

@KarlComSe, thank you for your report.
We've created internal ticket(s) MAGETWO-82696 to track progress on the issue.