Album covers are being blurred since iOS 12 installation

Hi everyone, I've issue with album covers since I installed iOS 12 to my iPhone 6 plus. The covers of songs/albums that aren't bought directly from iTunes Store are being blurred out when I'm listening them. I usually copy-paste the cover of albums from Internet, which I imported from other media (like CDs). Covers are downloaded automatically with the songs bought from iTunes Store. I had no problem with the covers downloaded from the Internet until I installed iOS 12. After installation, those covers started to blur, showed in the screenshot below.

However in the music, the cover is displayed as it should be seen (screenshot is below). I couldn't figure out why it's normal in the list bu blurry "now playing" tab

The cover of a song bought from iTunes Store is normally displayed like in the screenshot below.

Is there any other issue like this one? Or am I the first one to encounter this problem? I'm not sure calling this issue "a problem" because I couldn't understand whether Apple did this intentionally or not. If this is not done intentionally, what can be the solution for this? I couldn't found a way for solution or understood it why. Those album covers are displayed normally in my old iPod Touch 3rd genereation (iOS 5.1.1)