Symfony 4 : setting {_locale} parameter in route definition leads to "no route found" error

Question

I've a Symfony 4 app that is running fine until I want to define a route with the _locale parameter.

I define two routes for the same action with the following code :

/**
 * @Route("/sites/{id}",
 *    name="site_get",
 *    methods={"GET"},
 *    requirements={"id": "\d+"},
 * )
 * @Route("/{_locale}/sites/{id}",
 *    name="site_get_with_locale",
 *    methods={"GET"},
 *    requirements={"id": "\d+"},
 * )
 */

When I call the url /sites/1 I get the resource but if I call /en/sites/1 I get a 404 error "route not found".

Here are the things I've tried :

• tried to change /{_locale}/sites/{id} with /{locale}/sites/{id} and the route is working but, obviously, the locale is not changed.
• tried to set requirements for the _locale parameter but it is no better
• tried to set the {_locale} parameter as a prefix (in annotations.yaml) with no luck neither
• I can see the route definition in the debug:router :

site_get.0                                      GET         ANY      ANY    /api/sites/{id}                                                 
site_get_with_locale.0                          GET         ANY      ANY    /api/{_locale}/sites/{id}

Edit 1 : In the profiler for the given failed request /en/sites/1 :

• the configuration sections shows me thet the Intl locale is fr.
• In the routing section I don't see the same at in the debug:router command : {_locale} is changed into 0 (even if I set requirement for the _locale to fr|en)

Edit 2 : I was using Symfony 4.4.8 but downgrading to 4.4.7 seems to work. I also have the problem with 4.4.10

Any idea of another path to explore?

Thank you,

Vincent