What kind of files are we talking about? MP3? M4a? Flac? WAV?
What kind of tags are in the files? APE? ID3v2? ID3V1?
Are the files OK (which means have you checked them with at least one of the tools linked here: How to check files for errors?
And OK does not mean "they play" but that the integrity has really been checked with the tools.
There are car players that search the internet on their own for covers. Sometimes they find some, sometimes they don't.
Use the forum search function with the search words "car", "Cover", "art" and look through the threads.
I found at least 4 dealing with the same topic, unfortunately, none came back with a solution, so apparently the problem solved itself.