TW,
Here the issue is suspect is...when the confirmation was blocked...the confirmed quantity will be zero. After releasing the document in VKM3...the quantities will not be confirmed automatically. When he is changing the quantity, the availability check will be triggered again (and the credit check also) and confirming the quantities since the credit block is not applied for a quantity less than 11500. This is just a pointer for the OP to check.
Just like changing quantity to 11496...he should also check what is happening if the quantity is changed to 11501.
Also, if the reason is due to ATP quantities, normally system should create multiple schedule lines confirming quantities for different dates. This point can be analysed in detail if the above check fails.