How to reproduce : 

1/ Create a new newsletter, update it and add some line and colum to existing table : 

2/ In the 3 columns on the left, add 3 articles fount by "last publication" button. Save it : 

3/ Edit the newsletter, and in the right add 1 other articles fount by "last publication" button. Save it. 

=> Illustration of already existing article has been updated with illustration of newly added article. (see below)

 Note : It is not linked to newsletter, I reproduce exactly the same problem on an article.