at this page: https://www.ak-traditionelle-textilien-mv.de/newsletter-archiv/ I can see the two latest newsletters being shown, while the oldest one (08.05) is not. I’m not sure why it isn’t showing all three newsletters, but as long it is present in the database, it should show up.
Try to go to Newsletter Archive settings, change the “Showing the newsletter” option to something else, save, and check if the third newsletter is now showing up.