Just an observation really rather than a genuine concern...
I have BA booking ex-DUB to MCO via PHL, then on the return SFO-DUB via LHR/LCY. It's all on BA flight numbers and a single 125- ticket, but DUB-PHL and PHL-MCO are on AA metal.
Interestingly, myflights is showing completely random seats for us on the AA sectors, though when I go into aa.com directly with my AA record locator, 1A & 1B are showing correctly

(I phoned up and got these assigned a couple of days after making the original booking).
I assume the mismatch is because the app took a cut of the original booking but is no longer allowed to look at it ?