I went back and forth with them a few times on this, and they stood by the fact that AA6xxx is not in the valid range, despite the codeshare on the flight.

I flew one of the same flights again this past Sunday, and it posted just fine to AS:

08/14/2016 AMERICAN AIRLINES Flight 6019 IAH LAX Credited 345 780 1,125


So maybe they're starting to clean this stuff up. It's still not listed in the flight number ranges, but it's sold as AS, so it's clearly a partner.