Any airline re-booking you on another alliance if you have not left on your first leg almost never happens anymore. Seats are just too tight. You are presumed to be able to just go home and try again later. Not saying I agree with the policy. But it is not exclusive to American.
I will concur with the idea stated above that life is less complicated if you book end-to-end directly with a single carrier. The alternative is one of the few remaining real travel agencies that you pay for service. You have fallen into the twilight zone that is online travel agencies.
Last edited by jayer; Aug 7, 2021 at 4:02 pm