Substitute Material Issue in Subcontracting Purchase Order

Hello Experts,

We are into support project. We have issue with substitute material. We have material say "A" for which substitute material is "B" and the material "A" is not used now.

The material "A" is included in the material BOM "C". So final material is "C", for which component material is "A" which is NOT used now and has substitute material "B" is active for the current period.

When we create subcontracting PO for the final material "C", the component material is always picking as "A" though which is NOT used now

Could you please guide why the substitute material is NOT picked in the subcontracting PO component material list will be provided to the SC vendor?

Thanks in advance for your support.

Regards

Bala