Was pulling my hair out until I realized this was a bug! Not sure how to submit a bug report so I’ll post here.
I am making a Page that I set to have a Parent. The Parent page is using masonry inside columns and it works fine. The Child Page, displays the masonry at full width, inside a column (it’s almost exactly the same as the Parent, just a different category).
So I removed the Parent page option and made it it’s own standalone page, and the masonry displays at the correct width. I’m able to recreate this. So it works only if the Page isn’t a Child Page.