When using SearchService's findContent() with the new implementation (EZP-23524) using field filters, the field filter "useAlwaysAvailable" defaults to false instead of true.
Steps to reproduce:

1. Ensure a installation without demo content with eng-GB as default language and fre-FR as secondary language;

2. On legacy admin, at "Home", create:
folder "Folder" in eng-GB
folder "FolderFR" in fre-FR
folder "FolderEN" in eng-GB

3. Use on a custom bundle the following code to perform a search query:

$query = new Query();
$query->filter = new Criterion\Field( "name", Criterion\Operator::CONTAINS, "Folder" );
$searchResult = $searchService->findContent( $query, array( "languages" => array( "fre-FR" ) ) );

/* Results */
foreach( $searchResult->searchHits as $searchHit )
{
	$output->writeln( $searchHit->valueObject->contentInfo->name );
}

and verify that you'll get only the following results:

FolderFR

whereas you should get instead:

Folder
FolderFR
FolderEN

since, by using

$useAlwaysAvailable=true

or simply omitting it from the array at:

findContent( $query, array( "languages" => array( "fre-FR" ) ) )

it should add the default language 'eng-GB' to the array of searchable languages and thus, retrieve results found on both fre-FR and eng-GB languages.