XML results are overridden when multiple suites are executed

I'm running my TestNG tests Programatically. I'm Injecting multiple suites and executing. But when I execute the following, testng-results.xml doesn't contain all the results from each suite. I only get results of the last suite. Has anyone faced the same issue before?

            TestNG tng = new TestNG();
            List<XmlSuite> suites = new ArrayList<>();

            for (Deployment deployment : deploymentList) {

            tcList = tlsite.getTestCaseClassList();

                XmlSuite suite = new XmlSuite();
                suite.setName(deployment.getName());
                XmlTest test = new XmlTest(suite);
                test.setName("AutomationTests");
                List<XmlClass> classes = new ArrayList<>();

                for (Object className : tcList){
                    try {
                        classes.add(new XmlClass((String) className));
                    } catch (TestNGException e){
                        log.error("Error occurred while adding the class : " +e.toString());
                    }
                }
                test.setXmlClasses(classes) ;
                suites.add(suite);
            }
            tng.setXmlSuites(suites);
            tng.setOutputDirectory("my-out"); 
            tng.run();