Finally, I found the solution. The problem is with the RK3188 Car Head Unit; by default, it starts playing the media player. I don't know the reason why Poweramp is affected. I found the solution here --> SOLVED
Same problem in my case. I have an Android Car unit rk3188; I use Poweramp because it is a fantastic music player with a lot of features (Covers, connection with last fm, quality of sound..). But the problem is the autostart, no way to disable it. Try Tasker to kill the app whem start, but it takes some minutes to end this task. I am "walking" trying to find a final solution to this problem.