not urgent, but I'm curious – text inset apparently rendered as graphic?

The cover page for our product manuals has (as you'd expect) a different title and picture for each product, and uses a text inset for the standard disclaimers. In just one instance, I've saved as .pdf and ended up with a file where the disclaimer text is not searchable and cannot be selected; but text below the inset (same page) behaves as normal. Any ideas?