Many users have discovered that the notification LED does not work as expected. In applications such as WhatsApp, which allow you to choose unique colors for private or group messages, the LED does not reflect this configuration. In some cases, the notification LED shows a standard color regardless of whether we have set different colors for individual applications. This seems to be one of the most common problems of Galaxy S9 and Galaxy S9 Plus that users have detected.
In the case of WhatsApp, go to Settings> Applications and scroll down to WhatsApp. In the Memory section, touch Clear cache. Next, we start WhatsApp, open the configuration menu and configure the LED color in None. Finally, go to Settings (phone settings)> Display, disable the LED indicator and enable it again. We go back to WhatsApp and configure the color of the LED we want and it should work.
As for other applications, we may need to use a third-party application such as Light Flow Legacy until there is a permanent solution available from Samsung.