The companion voucher is irrelevant.

With IB, it doesn't matter whether individual legs can be booked using miles. You need to have availability for the married segments.

I can only speculate, but... There is some anecdotal evidence suggesting that married segment award availability is connected to the underlying paid fares. So a competitive one-stop origin / destination combination has award space in J but a quasi-monopoly original / destination combination doesn't. Since no other airline offers a J fare to XRY AFAIK, then IB is free to charge whatever it wants in J --> the algorithm doesn't allow J awards because it would rather keep the seats unsold in order to charge a high cash fare. (remember, this is all algorithm logic, because humans aren't involved on a daily basis in setting fares or award availability)

FWIW, there's not much point in paying the extra Avios for J - even with a 2-4-1 - on MAD-XRY UNLESS you have no other way of accessing the lounge in MAD during your layover.