The other bug is that even when you set correct size for an image, it displays wrong as you can see with the first image of the gallery.
I tried to remove, re-import but it does not change, although it displays correctly in layout with the correct size but fails in preview. I made a little screen movie:
I keep the post because it is interesting to understand what happens with the Masonry Container but I finally changed the Gallery and made it with Iconic Gallery which works smoothly.
Some of the original bricks supplied with Blocs seems to have not been updated according to the versions of Bootstrap.
That’s what I found regarding the columns management for the Masonry Bric. It still use BS 4 CSS class (card-columns) although BS 5 promote CSS-Grid or Flex (row-cols-*).