This happened to me when upgraded from my x to 11 upon my first sync with iTunes. And again this week with my new 14 plus. I looked up every possible solution offered on this site and opted for this: Go into settings, music and downloaded music. Then edit and delete all songs. After this, delete the music app from the phone, hard reset the phone, redownload the music app from the app store and re sync the music library onto iPhone via iTunes. It's not done syncing because I have over 13 000 songs to load but so far, every album I noticed that did not have correct artwork has corrected itself. Hoping this holds true for all albums. Good luck to you... I'm reading this has been a common problem for years.
Side note: some of my movies are also displaying the wrong artwork. I will be trying this same method with the movie app as well.