I wonder then if it’s a CD you bought that had autorip which also would have made a lossy version available for access//use online by Alexa, for example.
For the Gary Clark track you mentioned (which exact one was it so I can try?), try deleting that from your music/library in the Amazon music app. If it’s a track that is still in Unlimited you should now get the HD version. The downside will be that that track will also disappear from any playlist you have and you’d need to re-add the Unlimited HD version.
If this does work, I think there’s a bug in Amazon Music where it’ll offer up the lossy versions in your library rather than the HD version of that same track. I used to think it did the exact opposite…