That problem sounds like mine, I got an iPhone X with iOS 12 and a new Mercedes A-Class. The sound over BT jumps between muffled and clear, but its not only Spotify but also any other music-apps like Apple Music, SoundCloud or random videos on FB2Mate. So if its the same with the music app for you, I guess its some problem with our phones. The sound is perfect if I stream Spotify over the USB-cable to my car and the sound over the cars radio is also fine. I still have to test another phone to sort out if its a problem with the iPhone or the BT connection but at least in my case I'm pretty sure its not Spotify.
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