I think at this point it's fair to say that any type of audio quality loss is classed as a bug and needs to be fixed, regardless of how it presents itself. It wasn't there in iOS11 and is all of a sudden present in iOS12 so it's clearly a software issue. All the music I'm trying to play has been purchased directly from the iTunes Music Store and I'm listening to it offline directly from the local storage on iPhone via bluetooth in my car. These issues should not be happening, it was present in the beta, and it's present now on the final release after what Apple call 'extensive testing'.
I'm so glad I found this thread. I'm a bit of an audiophile, and I noticed a few weeks ago that treble, in particular cymbols (for whatever reason), click in and out randomly. I have been going crazy trying to figure out what the problem was. First I thought it was Spotify, but I was able to replicate the issue on youtube and Napster (using a family member's account). Then I thought it was my phone (iPhone X), but again was able to replicate the issue using a different device (iPhone XS).
I noticed just yesterday that plugging my iPhone into the USB port on my Jeep, connecting to carplay and playing music through carplay fixes (or circumvents?) this issue.
For a few weeks I thought it was possible that one of my speakers had blown, or that the UConnect system in my Jeep was having issues. My problem is not easily detectible since it's mainly background treble and I suspect a good amount of people would not notice it so it is comforting finding this thread.
Many thanks for the above. It once again confirms that the problem is an interaction between iOS 12 and Bluetooth, since a number of people have found that the problem goes away with use of an auxillary-type cable. As related above, my problem as well coincided with occasional high-treble sounds to such an extent that I suspected a defective speaker and even tried setting the EQ at Treble Reducer (Settings>Music>Playback). Clearly this is something that Apple has to fix, but on the multiple occasions I've contacted them, they're either clueless or insist that the new 12.0.1 update addresses the problem, which by virtually all accounts it does not. Until then I'm sticking with 11.4 as long as possible.