Hi,
It's not a bug. It's just that there is no mechanism to search through the menu items you created to see if there is one using that brand as main category to use it.
Usually merchants don't create a menu item for each brand so we didn't add the code to do that search. First, it takes time for us to work on that, then, it wouldn't be much used, and finally, it adds processing and an extra MySQL query while most merchants won't need it.
Now if you want to use the same link, you can enter the main URL of your brand in its "canonical URL" setting.
Then, make sure that the "force canonical URL on listings" setting of the HikaShop configuration is set to "Use canonical URL if specified" and the system will use that URL on the product page instead of generating one dynamically.