I have seen this type of thing before but it was in the context of booking partner awards. If the airline has not filed a fare between your origin and destination, the system treats it as two separate flights requiring two separate awards. In my case, I could not book a single award for LED-LHR-BRU because BA did not offer a LED-BRU fare. But that should not be an issue in this case. AA definitely offers MAF-DEN fares. I just verified that with ITA. I think the best next step is to call back. AA may have an error in its system but they should be able to fix it.