How do I deploy a Page Manager page using code?
aha. This problem was caused by a forgotten dependency within the Page Manager page. Instead of using a default layout such as 'two column,' I'd used the Bootstrap layout provided by the Panels Bootstrap Layouts, and then forgot about it. Features didn't add Panels Bootstrap Layouts as a dependency, it remained disabled after deployment, and the dependent page content didn't show. After I enabled Panels Bootstrap Layouts, the content items showed right up within the variant's Content tab.

So if your feature or your Panels/Pages import doesn't recreate the entire setup, review the original for any options that might be provided by another module.