Same issue using my Motorola G6+ after android 9 update connecting to a 2016 VW Golf via Bluetooth using Poweramp 3 build 828. The handset will connect to the car fine however as soon as Poweramp attempts to play (either requested by the car or started on the handset) the handset Bluetooth crashes. This issue then loops until something else on the handset takes audio focus.
All other applications I have tried can still play to the car fine (Youtube, Google maps and Rocket player). The issue is definitely linked to Poweramp as I can trigger it at will by giving Poweramp audio focus and fix it by moving audio focus to another application. Working around the issue by using rocket player for the moment in the car but would very prefer to be using Poweramp as its a better app.