$interface = UserInterface::class;

// current method : that won't work
$em = $this->doctrine->getManagerForClass($interface);
$io->writeln("Manager (method 1) : ".($em ? get_class($em) : "not found"));

// second method : that works

foreach($this->doctrine->getManagers() as $em){
    if ($em->getClassMetadata($interface)) {
        // founded !
        $io->writeln("Manager (method 2) : ".get_class($em));
    }

}

 public function dehydrate(object $object): mixed
    {
       // if the object manager is resolved, it will provide the right metadata for the final class 
        $id = $this
            ->objectManagerFor($class = $object::class)
            ->getClassMetadata($class)
            ->getIdentifierValues($object)
        ;

 public function hydrate(mixed $value, string $className): ?object
    {
        // an empty array means a non-persisted entity
        // we support instantiating with no constructor args
        if (\is_array($value) && 0 === \count($value)) {
            return new $className();
        }

        // e.g. an empty string
        if (!$value) {
            return null;
        }

        // $data is a single identifier or array of identifiers
        if (\is_scalar($value) || \is_array($value)) {
            return $this->objectManagerFor($className)->find($className, $value);
        }

        throw new \InvalidArgumentException(\sprintf('Cannot hydrate Doctrine entity "%s". Value of type "%s" is not supported.', $className, get_debug_type($value)));
    }