Diagnosis
Unfortunately, yeah, it's a common gripe with the Highland. The firebox gets much hotter than the cook chamber, and the standard black paint isn't always up to sustained, high-temperature use.
How to Fix It
Let it cool completely, then scrape off all the loose, flaky paint with a wire brush or scraper. Once you're down to bare metal or stable paint, you need high-temperature paint. Get a spray can of Rust-Oleum Specialty High Heat (rated for up to 1200°F). Apply thin, even coats, letting it dry between coats. Then, you need to cure it by running a very small, gradual fire to bake the paint on before a full cook.
If That Didn't Work
If it keeps happening, the underlying issue might be that you're running too hot a fire for too long right against the firebox walls. Try building your fire more towards the center of the box, not pushed up against the sides.
Heads Up
This is purely cosmetic rust protection. Don't use any paint on the interior of the cook chamber or firebox—only the outside. And do the painting in a well-ventilated area, obviously.