[onewaytrigger]

Unfortunately the AS logging was disabled when you ran that. I have tried to duplicate this on my end, and when I have a featured artist is the title the first one works, then the next one with a featured artist does as well. It might be worth checking userdata/addon_data/script.artistslideshow/merge to see what's in there. That's where images are put when there are multiple artists to be found. It almost sounds like that directory isn't getting cleaned out properly. Since it works for me here, if there are images in that directory when Kodi isn't playing any music, you might check to see if the permissions are set correctly so that Kodi can delete from there.

Here's the log with AS logging. http://pasted.co/2b31a179 (Sorry couldn't post on pastebin due to limitation)
I'm on Win 10, the Kodi is stored on default folder ("\UserName\AppData\Roaming") like many other aplications, should permissions be a problem? Some times Kodi delete the fanarts imediatly, sometimes them are not deleted but replaced when another multi artist song is played. In any case the odd mix of fanarts stills, but the wrong ones are not stored under "script.artistslideshow" folder.
I performed a search by picture on the whole Kodi folder (since is almost default) and and artist fanarts are stored on "userdata\Thumbnails", why AS is picking them from there?

I provided some screenshots:

It depends on the skin. AS just dumps all the images for the two artists in one directory. The skin actually decides how to display it.

Is there any limit to images dumped folder "merge"? I ask because as you can see on first screenshot. AS picked full 20 for Charli and none for Azlea.