The closest I've found is this thread https://forum.powerampapp.com/topic/15623-pa-does-not-remember-the-position-of-the-track-when-turn-it-off/?tab=comments#comment-64657
while a very similar situation (my car's head unit is android based, I just called it a tablet for convenience), this user had no track position remembered at all. Mine is not remembered only when using USB storage. I'm eager to learn where the last position data is stored if anyone knows. I have a feeling that perhaps there's a relationship between the USB interface initializing late and Poweramp trying to read data that isn't available yet.