magento2: Wrong drop-down option labels for configurable products attributes using custom source

Preconditions

1. Magento 2.1.1 (updated from 2.0.8)
2. PHP 5.6.25
3. MySQL 5.6.23

Steps to reproduce

1. Create a module with a custom Model
2. Create an attribute for the product entity with a custom source referencing the new Model
3. Create an attribute set with the new attribute
4. Create a configurable product using the new attribute set and a few associated simple products
5. While ererything is working fine, update from 2.0.8 to 2.1.1

Expected result

1. On the backend, the product edit pages should show options for the dropdown that are returned by the getSpecificOptions() of the class extending AbstractSource
2. On the frontend, the product view page should show only available options of the simple products associated to the configurable product

Actual result

On the backend product edit page, everything seems all right, all options are loaded from the custom source / database table

On the frontend, the configurable product drop-downs show the correct attribute name and the right amount of options for each attribute, but the <option> labels and values show wrong data, like “Male” and “Not Specified” or even “null”

This error also appears on the product configuration modal (backend / new order).

Inspecting the code of the product view page, I’ve found this part where we see the wrong option labels being applied (the attribute labels are correect, only the options labels are wrong):

<script type="text/x-magento-init"> { "#product_addtocart_form": { "configurable": { "spConfig": {"attributes":{"286":{"id":"286","code":"ticketsale_session","label":"Session","options":[{"id":"1","label":"Male","products":["72","73","74","75"]},{"id":"3","label":"Not Specified","products":["76","77","78","79"]}],"position":"0"},"287":{"id":"287","code":"ticketsale_sector","label":"Sector","options":[{"id":"1","label":"Male","products":["72","73","76","77"]},{"id":"4","label":null,"products":["74","75","78","79"]}],"position":"1"},"288":{"id":"288","code":"ticketsale_ticket_type","label":"Ticket Type","options":[{"id":"32","label":"Full price","products":["72","74","76","78"]},{"id":"33","label":"Half price","products":["73","75","77","79"]}],"position":"2"}},"template":"R$<%- data.price %>","optionPrices":{ /* removed for simplicity */ },"productId":"71","chooseText":"Choose an Option...","images":[],"index":{"/* removed for simplicity */}, "onlyMainImg": true } } } </script>