I'm with the OP, I had the similar issue back in Feb 2017, my return was LIN-MAD-MIA, all IB flights with AA codeshare and I remember AA had a schedule change on the outbound (USA-Milan on AA) so I got reticketed. I was able to select my seats from AA website (after a redirection to IB website) but I was unable to check in at T24, I called AA and they said everything was good but when I arrived at the Airport IB was unable to check me in because it was missing the eticket or if was not confirmed. I called AA and they said everything was confirmed, and told them to check with Oneworld Desk, they did that and they found I was not confirmed with IB, so the agent re-ticketed and I should get an email in less than 20 minutes, that never happened (sometimes take more than that), so I called again and the agent call to another department to do it right away. I was lucky because I arrived on time and I was not checking any luggage, but I almost miss my flight because the cutoff time for check-in.

So for those who said you need to call IB, they will tell you that you have to call AA and most of the time in this cases, they will say everything is ok.