Shipment number missing when using invenory commitment preference

Hello

Could you please help me with the below issue?

we have two branch plants A and B. System should first check if the inventory is available in A, if the inventory is not available or if it is partially available it should check in B and commit the inventory accordingly. For example if the inventory is not available in B as well, then it should backorder the quantity in B. This is achieved using inventory commitment preference.

Here is the scenario

Item X - Availabe inventory in A: 6; Available inventory in B: 0

Item Y - Availabe inventory in A: 0; Available inventory in B: 50