Part to be discontinued is not redirected to follow-up part sometimes

Hi, friends

In my case, requirement for the part to be discontinued is not redirected to the follow-up part in production order sometimes.

When creating production order, if I entered order quantity which requires the to-be-discontinued part more than its stock, the requirement can be redirected to the follow-up part correctly.

If I entered order quantity which the stock of to-be-discontinued part can cover the requirement, of course the requirement will not be redirected.

But after that, no matter if I saved the order or not, if I change the order quantity to a big number, though the stock of to-be-discontiued part is far from enough, the requirement is still not redirected to the follow-up part.

Why does this problem occur?

Thanks.

Wesley