I took some screenshots to better illustrate this behavior:
Here, MMA main interface, with correct metadata and AlbumArt:

In the homescreen widget, AlbumArt and metadata are also correct:

Here, we can see the widget in the notification shade. Metadata is correct, but the AlbumArt displayed is the one from the previous song:

Here, the same thing for the lockscreen widget. Correct metadata, but incorrect AlbumArt. Also, note that the background fullscreen AlbumArt is correct:

I noticed this behavior in two different devices: OnePlus 3T with Android 8.0 and Samsung Galaxy J5 with Android 6.0.
Sometimes, this doesn't happen right away. It needs some time of playback, so you can notice it.