This has come up before in the past, notably in a post I started as I experienced the same thing several times. No definitive answer from AA and lots of "opinions presented as facts" from folks here, but at the end of the day, the "ghost" in the middle seat followed me around the cabin for long enough I was convinced there was some sort of courtesy block happening. I managed my expectations accordingly that the seat would be filled (since there was no policy guaranteeing such a block) but also stopped trying to escape the ghost.
Ultimately I think the flights ended up going out full in most cases so the block didn't hold.