It seems that when setting a scoped option, there are set*() calls on the global LocaleConfig, which override the global config.

So the following:

// assume originally set config is `'supported_locales' => ['en']`

// add a route with scoped config for English and Dutch
Route::localized(function () {
    Route::get('with-scoped-config', function () {
        return App::getLocale();
    })->middleware(['web', SetLocale::class]);
}, [
    'supported_locales' => ['en', 'nl'],
]);

// add a route without any config
Route::localized(function () {
    Route::get('without-scoped-config', function () {
        return App::getLocale();
    })->middleware(['web', SetLocale::class]);
});

would give the second route the same config as the first route.

also, calls afterwards to LocaleConfig::getSupportedLocales() and LocaleConfig::getOmittedLocale() would retain the value of the last route registration with a corresponding scoped config.

It seems the two lines can simply be removed, but might be worth checking if they had another purpose?

added test that failed before the changes to LocalizedRoutesRegistrar