You can change the blog sidebar layout in “Appearance > Customize > Layout” — that worked, thank you!
Now I get two columns of masonry without any shift, but if I include the right sidebar the shift occurs with 5 posts. You can see what I mean at http://www.kcsanews.com/kcsa-blog — I have restored the sidebar for now.
I would also like to get three columns with a screen>1100px wide (My container width is 1100px). I have all Posts with Masonry Width set to “Normal”. Changing all to “Medium” doesn’t make any difference.