Months ago, I posted in support about needing help with showing my posts on the posts page in a masonry style without sidebars. We weren’t able to resolve why it wasn’t working after a few attempts. I just discovered something that might help to find a solution.
If my page (called “blog”) is not set as “Posts page” in the Reading section of Settings, then it displays correctly. After setting my Blog page as the Posts page, it shows the sidebar. If I switch the Posts page to another page and then refresh my browser, it shows the Blog page in the masonry format with sidebars (which is the format that I desire).