The problem appears to vary along with the solution, where there are some users who are claiming that the issue has been resolved following a factory reset, indicating that it might be a software problem, while others have tried similar solutions with no results, suggesting that this might be a hardware problem instead.
Based on this it’s hard to say what the root of the problem could be, or if these might be two separate problems that coincidentally affect the touchscreen on the phones. However the good news is that Samsung is looking into it. According to the company, “At Samsung, customer satisfaction is core to our business and we aim to deliver the best possible experience. We are looking into a limited number of reports of Galaxy S9/S9+ touchscreen responsiveness issues. We are working with affected customers and investigating.”
No word on when a resolution will be offered and whether the cause of the problem will be revealed, but hopefully it will be resolved soon.