I thought this was interesting. I just purchased MCT-(BA)-LHR-(AA)-JFK-(AA)-LHR-(BA)-MCT in J for the Christmas and New Years holidays. (I live in Abu Dhabi, but flying from MCT in J often is a lot cheaper than from AUH.) After ticketing, I called AA to apply VIP upgrades for the LHR-JFK and JFK-LHR segments. Both were showing A1 on ExpertFlyer, so I knew both would clear -- which they did. Out of curiosity, I checked the flights again after hanging up with the agent, and both are still showing A1. I have logged into my reservation and have confirmed that the agent properly applied the upgrades. So perhaps this is simply an example of AA doling out the A inventory one seat at a time, though I don't recall seeing or reading about this before. (FWIW, the other LHR-JFK 77W flight on my travel date is showing A5, and the other JFK-LHR 77W flight on my travel date is showing A2, so it's not the case that this is SOP for all flights.) I'll check the inventory again after the ticket is reissued, but the inventory should be pulled when it's allocated, not when it's ticketed.
[Edited to add: ticket has been reissued, and both segments are still showing A1.]
[Edited to add: ticket has been reissued, and both segments are still showing A1.]