I just had this issue yesterday with my Pixel 4a. I tired rebooting the car and phone without success. Luckily my wife plugged in her Pixel 5 and that worked.
It's been so unreliable lately especially when using Waze. Plus reporting via voice on Waze hasn't been working for ages. Add the intermittent disconnections and I'm tempted to buy a cheap iPhone for Carplay when driving
I've been having intermittent connectivity issues, it typically requires that I restart my p6p to resolve. For whatever reason there are times that AA just won't connect properly... Or at all