Shipment release job works incorrectly with backorders

Description

Precondition: A variant, "A", with In stock = 5, Back order = 5.

Steps to reproduce:

1. Go to Commerce Manager.
2. Create Order1 with variant A, quantity 5 --> Order1 is submitted with In stock quantity.
3. Create Order2 with variant A, quantity 5 --> Order2 is submitted with backorder quantity.
4. Add payment for Order1, Order2 --> Release shipment button for Order1 is enabled, for the Order2 is disabled.
5. Run the Shipment Release job.

Expected: Only Order1 is released.
Actual: Both orders are released.

Note: After order is released, user cannot delete order2. It displays this error:

Failed to process item(s). Error: Workflow generated an exception, please look at the previous error for more details