I’m curious if it is a potential issue for API platform?
As you know API platform has State Processors

But if you’re using state processors the validation won’t work.
Here is my code:

public function process(mixed $data, Operation $operation, array $uriVariables = [], array $context = [])
{
    if ($operation instanceof Post && $data->getPlainPassword()) {
        $hashedPassword = $this->passwordHasher->hashPassword(
            $data,
            $data->getPlainPassword()
        );

        $data->setPassword($hashedPassword);
        $data->eraseCredentials();
    }

    return $this->processor->process($data, $operation, $uriVariables, $context);
}

In my User entity I use #[UniqueEntity('email')] constraint.
So if I won’t use Symfony Validator in this processor the exception would be thrown.

An exception occurred while executing a query: Duplicate entry '[email protected]' for key 'user.UNIQ_8D93D649E7927C74'

So the solution is:

$errors = $this->validator->validate($data);

if (count($errors) > 0) {
    throw new ValidationException($errors);
}