This is my understanding as well for awards on AA metal. For awards on US metal, the corresponding codes are I and O for business and first plan aahead awards.

After exploring further, it seems that the regular AAdvantage engine in some cases allows bookings of awards AAA-BBB-CCC where there is award availability on AAA-BBB and BBB-CCC even if there may not be availability on AAA-CCC. The meeting services desk is enforcing the married segment rule in these cases and not offering the award on AAA-CCC.

Additionally, it looks like the agents are poorly trained in looking for the correct inventory ex-MEX (they are looking for I instead of O, because they think the MEX-XXX segment must be flown in business on US, but US still markets those flights as First).