Source Based Promising does not honor total availability quantity while promising

Summary:

Hi Experts,

I am doing a POC on source based promising for one of our customers. I am noticing an odd issue, which I will try to explain here.

A) BUY1 is "source based promising" enabled item.

B) It has 100 quantity available for reservation/transactions.

C) When I do a quick availability check for 100 quantity while adding the item in the Sales Order, GOP shows "Low Inventory, 99 quantity available".

However, 100 quantity was up for grab.

D) I added the item in a Sales order line and submitted it. The requested date was 17-Sep-2025.

E) GOP split the line as below: