If a file contains multiple artists then thumbs are not displayed.
The issues appear to be in how the thumbnails are named when MP creates them.

if you have two artists then the call at the end of OnRetrieveCoverArt
strThumb = Util.Utils.GetAlbumThumbName(tag.Artist, tag.Album);
is called with tag.Artist set to "artist a | artist b"

However when the thumb is created, it is named "artist a;artist b-album" therefore thumb is not picked up in the above call.

There is also a second bug if artist a starts with one of the values we strip off. eg. I tested on a file where artist is set to "The National" and I have turned on the option to strip The, Les, Die... I then added a second artist of test so in ID3 tag the artist is "The National;test" which is picked up correctly when browsing views etc. When thumb was created however it was called "National;test, The-album". So the "The" was stripped from artist a but appended to artist b