src/Exceptions/ApiExceptionHandler.php
public function handleValidation(ValidationException $e)
    {
        $e = new CustomValidationException($e->validator->getMessageBag(), $e->getMessage(), $e, [], $e->getCode());
        return $this->handle($e);
    }
public function handleValidation(ValidationException $e)
    {
        $e = new CustomValidationException($e->validator->getMessageBag(), $e->getMessage(), $e, [], $e->getCode());
        return $this->handle($e);
    }
Dingo API does not handle default Laravel validation exception.
Wrap it into ValidationHttpException, processed properly

@param ValidationException $e Original exception to wrap and handle
@return Response