I don't have an explanation, but I've noticed similar behavior when I'm trying to force "abnormally" long layover.
Your flight has the same issue - it is a >9 hr connection. If you choose the 49 min connection; or the 3 hr 39 min connection, then it does properly link to AA.com. However, the 6'29" and 9'21" both say "Check price". Furthermore, those flights do not defaultly show up on an AA search.
BTW, I had to break it up into a multi-city itinerary with choose by schedule and selected the individual legs to purchase flights like this. It will show the correct price.