WIP Supply Sub Inventory Issue

Hello,

I am getting some issue with WIP Supply sub inventory.

Job is taking WIP Supply type from WIP parameter even it is defined at Item sub inventory level (Item level in tool)

My analysis : 1. At BOM level supply sub inv is blank

                      2. At resource level supply sub inv is blank

                      3. At item level  sub inv locator is defined

As per my understanding, if supply sub inv is not defined at BOM level it will go first to item level then will go for WIP parameter default.

This is not happening for all jobs for that assembly, it is only for few cases to taking sub inv locator from WIP parameter, I just wanted to know this behavior why it is defaulting from WIP parameter.