I’ve also tried selectOptionByValue and selectOptionByLabel, but Katalon is selecting the first value twice after it selects the second value. It should select 5, 6, and then 9.
Like @Russ_Thomas has said, we ideally need to see the HTML of the available options, before as well as after you select one of them. My guess is that maybe after you select an option from the list, that option then disappears from the HTML, which is throwing off your subsequent selections.
But again, we can only guess without seeing what you are working with.