Why hands free pause and playback button not working while playing youtube video ?

Samsung is aware of this symptom which may appear on some limited S9/S9+ devices and this symptom is caused by Google’s YouTube application itself. (YouTube version with issue : 13.14.55 )

Regarding this, Google will release improved YouTube version to resolve these symptoms. Samsung is working closely with Google to make updated application at earliest possible to prevent any inconvenience that consumers might experience.

Therefore please check regularly for updates to your YouTube application. Samsung always encourages users to keep your software and apps updated to latest version.