Taking your example of LAX flights for example, I have seen first hand a number of people flying the AA codeshare who have been shafted on seat selections where they rang AA (or other means) and had seats selected only to find at check in that these seat selections never stuck.
Wow, I had no idea - do you know what status these people had at the time? Surely they wouldn't bump a WP for say a non-CL due to being on a codeshare?
I have a LAX-MEL (AA7356) later this year as part of a LONE4 and my WP selection for 80A is still there..