If imported songs have filenames that include one or more apostrophes, album art will not be displayed when the song is selected/playing. Instead, it will display the album art of the previous song that was playing before it.
Apostrophes in the tags/description of the songs don't seem to have any bearing on this. Apostrophes in filenames don't appear to have any effect on any other feature within Spotify.
Remove all apostrophes from filenames. Afterwards, Spotify will correctly display the album art associated with the song.
This issue has been submitted a few times before over the past 3 years and still doesn't seem to be fixed: