Had the same problem - 2012 Prius, Stereo seemed to struggle to turn on until I unplugged my iPhone, almost as if the phone was trying to push something to the car...
Will just have to deal with not charging my phone in the car for the time being.
No big deal I guess, just surprised me when I was trying to get my music to play as I rolled out of the driveway and found my stereo was flashing and cycling power...