Given generated matching routes, it looks like the problem may be related to the _format parameter.
Could you please try to remove this from your config/routes.yaml file to confirm @Wirone?

@odolbeau Thanks, additional info:

Without defaults:

'/' => [
    [['_route' => 'app.foo.landing_page', '_controller' => 'App\\Foo\\LandingPageAction'], null, ['GET' => 0], null, false, false, null],
    [['_route' => 'app.foo.landing_page', '_controller' => 'App\\Foo\\LandingPageAction', '_locale' => 'pl'], 'pl.foo.loc', ['GET' => 0], null, false, false, null],
],

With _format: json in defaults:

'/' => [
    [['_route' => 'app.foo.landing_page', '_controller' => 'App\\Foo\\LandingPageAction'], null, ['GET' => 0], null, false, false, null],
    [['_route' => 'app.foo.landing_page', '_controller' => 'App\\Foo\\LandingPageAction', '_locale' => 'pl', '_format' => 'json'], 'pl.foo.loc', ['GET' => 0], null, false, false, null],
],

So that non-strict rule is always there as a first one.

FYI: I've run cache:clear after every change to be sure it's fresh. And removed en: 'foo.loc' from config so output is smaller.

@nicolas-grekas is it expected that we still have a route without any host requirement when we use locale-based host requirements ?

@xabbuh could you remove "Waiting feedback" label, since I've provided additional information 2 weeks ago?

@nicolas-grekas could you take a look at @stof's comment?

is it expected that we still have a route without any host requirement when we use locale-based host requirements ?

Nope, it's not. PR welcome.

Hey, thanks for your report!
There has not been a lot of activity here for a while. Is this bug still relevant? Have you managed to find a workaround?

Bug still exists in Symfony 5.2.6, here's reproducer → https://github.com/Wirone/symfony-issue-38209

Bug still exists on 5.3, I've pushed commit with bumped dependencies to reproducer.

It took me a while to figure this out, but there is no bug here:
the route is imported twice:

• once with host info thanks to your import statement in routes.yaml
• twice without host info because of your Kernel's $routes->import('../config/{routes}/*.yaml');