Hello,
Like some, I have experienced sporadic misbehavior, play interrupted, Android player taking over, etc. (My phone is Samsung Galaxy Note 2, Android v 4.3, Toyota Prius 2013). I am now going to the Android task manager and killing idle apps and clearing them from memory, on the guess that idle apps in memory may sometimes misbehave and ask for audio focus. This seems to make a difference, so just passing it along.
Try deleting the BT pairing from your device & the Prius, then re-pair again.
Things seem to have stabilized of late. I ensure that blocking mode is turned on and doing a reboot of inactive applications, clearing memory. That seems to help.
Thanks for the update