The behavior seems to be by design, and it has nothing to do with offline files.
When searching "Everywhere" (selected by default), Windows Search apparently displays only two results from each source (apps, settings, files, not sure what else).
By choosing "Files" from the dropdown menu above the search box, all expected results appear in the list.
I confirmed this on a different PC with "standard" (non-offline) files. While I consider this a serious design flaw regarding user experience (there should be something to indicate that not all results are shown), at least now I know how to find all my stuff.