Material allocation into alternative sequence

Hello,

we have the following business case.

There are 3 machines that are producing the same material but in the BOM we have a item (C) that will be used only in the machine 3. (we are using only one production version)

We try to allocate item C at the alternative sequence of the machine 3. But when we open the production order, it seems that the item C is allocate to the standard machine.

Can you please explain why is it not allocate to the alternative machine? is this not standard?

thank you!