That doesn't make sense to me. The compensation, as I understand it from your description, was for the involuntary downgrade. In such a case, you are also entitled to pay for what you got, not to pay for what they didn't provide.

You might find it worthwhile to send a note to (or call) mileage plus - keep it short and simple - figure out the difference between F and J on that segment and ask for corresponding mileage credit, explain circumstances of downgrade, don't mention the voucher.