Inventory tracking for backorders only gives the maximum available quantity, not the quantity requested by the user.

Expected behavior

Expect 20 item in cart, 19 of which should be backordered.

Actual Behavior

Got 1

Steps to Reproduce the Behavior

1. Create product
2. Add a top level variant
3. Add 1 quantity
4. Disable Deny when out of stock
5. Publish product and variant
6. Purchase 2 or more of that variant
7. Check cart has only one of the amount you ordered