I haven’t dealt with URL changes on Amazon listings myself. My guess is that Amazon doesn’t change the URL on older listings even if you change the title. To do so, Amazon would have to dynamically create a permanent 301 redirect every time a seller changed a title. This would be an SEO nightmare when you consider Amazon’s overall third-party usership. I can’t say that’s the exact reason, but that’s my best guess based on my own 301 redirect page migration experience.
You can tweak every other section of copy to reflect the brand name to maximize your search engine results if the URL change is a no-go. Good luck!