The default cover art for an unknown album is no cover.
Instead, you will get a picture of the “album artist” of that album.
So, it may be that perhaps the album artist got mismatched. Which in turn is usually caused by either a wrong/missing embedded meta tag for the album artist, or an improper folder structure where those albums are stored in.
See All my Various Artists albums show the album artist as John Williams in Plex - #4 by OttoKerner for a few pointers.
