Thank you.

When I logged into the US site and I saw the message saying I needed to go to the AA site, instead of clicking the link to continue, I manually went back to the AA site which didn't work obviously (habit of not clicking links these days unless needed).

There was indeed a separate PNR for the US codeshared flight that let me select the seats on my return flight.


Interesting, even after choosing my seats with the alternative PNR, when I log into MMB with the "main" code, it still shows my return flight seat as unassigned.