When creating new pages using a page builder, the layout expands to full width even though I have “default page builder container” selected and “1080px container width” set in the customizer.
I expect the content to be contained within a 1080px container, not to expand the full-width of the viewport.
The page has a body class of fl-builder which GP used to add when it detected Beaver Builder, only gets applied to really old pages. See this topic for the reason and fix: