But you're not really playing FLAC in the car?
Your phone is playing the FLAC and then just outputting regular headphone quality (i.e. poor) analogue signals for the car to amplify.
Playing a real CD will reveal best results, but admitted is a bit old hat these days.
I use FLAC at home, but just batch convert the whole 6000 tracks to 320CBR MP3 for the car, which is the top rate of MP3, and in the car this is imperceptible from a CD IMO.