As the Documentation says: “The default value of offset is -1, which defaults the switching behavior to standard. In this mode, the server determines a good transition point between the streams forward in time from the point it receives the switch call, and switches at that point.”
So you have to change the ‘offset’ parameter to a value higher than the current playback time (Netstream.time). If the value is less, a NetStream.Play.Failed status event is sent.