Every time different Album Art missing

I have a library with 62000 tracks and 3346 albums. All folders on my NAS have a Artists.jpg and cover.jpg. As soon as I add new content and need to reset the album cache, the problems occurs.

After every opening of Artist after a cache has been cleared, new Artist appears empty. So i add them to /data/albumart/personal/artist. Then do a cache clear and opening the page again, to find the next 20+ artist missing that where there before. By now the custom folder contains 1297 entries. Which I find a bit ridiculous and hard to maintain. Issue has passed the folder multiple times by now.