If they have retrieved your PNR still have it on their screen whilst doing the availability search for MADOPO, the system will consider that as married segment availability.
If you were to call up and on a fresh call and just ask for MADOPO availability, without mentioning that you hold and existing booking SCL-MAD, I suspect the availability will be there.

Alternatively, it could be that the availability is limited to specific office IDs, known more generically on here as point of sale availability.