You are using a child theme which sets the articles width to 49%. We’ve changed this value to 48% in version 1.7 for the Masonry articles alignment to work.
Since your site still has the old value, articles no longer fit next to each other and Masonry re-arranges them in one column.
You should only include the changed values in the child theme; there is no need to copy whole blocks of declarations for a single value.