It seems Shopify has rolled out some differences in the page component for Shopify built pages. Spacing being the main factor.
Using the <Page> component in our apps just has the Polaris-Page class. But checking Shopify pages, I’m seeing Polaris-Page Polaris-Page__PageBreadcrumbsExperimental Polaris-Page--fullWidth.
Is this something that will be rolled out to Polaris at all? I’m not too confident given the lack of activity from Shopify over on the Polaris github.