Here's my results. Before changing any settings, I tried PA with AA in another vehicle and it worked fine. Where it worked is connected to an aftermarket Sony head unit XAV-AX5000. I then went back to where it failed, my Ford Fusion Sync 3 head unit with AA support, and it still only played audio through the earpiece speaker. I disconnected, disabled Hi-Res for BT, tested again and now it works. Just to confirm, I re-enabled Hi-Res for BT and tested again and the problem returned. Switched it back to disabled and I'm all good. Looks like that was the fix! Interesting that it occurred only with one of my AA head units but not another.
A follow-up question though, what is the impact of disabling Hi-Res audio for BT?