In v3.x, if you create a role, add a child role too it, then add the parent role to the Rbac instance, the child role is not added. Using your own example code:

use Zend\Permissions\Rbac\Rbac;
use Zend\Permissions\Rbac\Role;

$rbac = new Rbac();
$foo  = new Role('foo');
$bar  = new Role('bar');

$foo->addChild($bar);
$rbac->addRole($foo);

echo $rbac->hasRole("bar") ? "works" : "doesn't work";

This displays "doesn't work". Similarly, attempting a permission check against role "bar" raises a "No role with name 'bar' could be found" exception.

This worked in 2.x