OP - I have this same issue with the latest Poweramp running on my Android 10 based iDoing tablet car head unit. My head unit is set to fully shut down when the car is off, but there seems to be some non-controllable delay before it does fully shut down (despite the screen and lights going dark), because wherever Poweramp left off playing audio, it will be a song or two ahead of that when I turn the car back on, so clearly it continues playing unknowingly for a few minutes while the tablet is in standby before it fully powers down.
I don't see any settings in Poweramp or in the HU to change this. Did you ever figure it out?