I have noticed this issue for several months in MC (if not forever). It essentially looks like MC doesn't consciously update the onscreen thumbnails. Take a large thumbnail, for instance, which is the most prominent place this is seen. When the thumbnail does not exist, it is generated. Unfortunately, each row/line/song seems to have its own part of the thumbnail. I have to roll over the rows to get each row to update with the new image. With the new (very useful) feature that adds space so large thumbnails can be the same size (only when grouped), it is more of an issue because there isn't anything there that can be hovered over.
They seem to update but it takes a while.
Is this problem reproducible? It could be partly due to my large cover art but I wouldn't necessarily think so since these are much smaller thumbnails being displayed.