Hi,
I solved the problem for this product. But I think that it will happen for other products too.
The problem is that the product has many categories affected, and this system take the first returned category, and for this product the first returned category wasn't the category with the id 118.
But the category 77, and in the category 77 it's the last product, that's why there wasn't the next button.
What you can do is order the categories affected to the product on the product edition page, to be sure that it will link to the product of his main category.