The issue was a slash at the end of the route. In CI3 we didn't have required routes, and when I made the route for this page I left a trailing slash at the end. Example is this:

$routes->get('manage/project/(:num)/dataset/(:num)/(:any)','Manage\Dataset::$3/$1/$2/');

had to be changed to:

$routes->get('manage/project/(:num)/dataset/(:num)/(:any)','Manage\Dataset::$3/$1/$2');