Custom module routing issue

Question

I've a custom module where I have create the following route:

my.custom.route:     
   path: /{someVariable}/my-module/test
      defaults:
        _controller: '\Drupal\my_module\Controller\myController::build'
      requirements:
        _permission: 'access content'

On the controller I've the following code

public function build($someVariable)
  {
    echo 'here';
    dump($someVariable);
    exit;

When I go to /devel/routes the route is there with the name I see all details of the route

when I call the path function it generates the route as expected, but when I go to the page just throw a 404 response instead give me the expected output.

if I change the path to have the variable in any other position apart of the first part of the url it works as expect

I'm working with Drupal 9.3.2

Thanks