I believe it would NOT be a let. That is, it would be a legal service and the server would earn the point (assuming the receiver did not return it).
I'm referencing the rules at PongWorld.com. In the description of the service (rule 2.06) the rules state (emphasis mine):
2.06.03 As the ball is falling the server shall strike it so that it touches first his or her court and then, after passing over or around the net assembly, touches directly the receiver's court; in doubles, the ball shall touch successively the right half court of server and receiver.
In describing a let (rule 2.09) the rules state (emphasis mine):
2.09.01.01 [The service shall be a let] if in service the ball, in passing over or around the net assembly, touches it, provided the service is otherwise correct or the ball is obstructed by the receiver or his or her partner.
In your description, the ball has already completed the "passing over or around the net assembly" and landed on the other side. As such, the serve is legal and what happens next is all part of normal play.
There is an exception in rule 2.09 if the player is in a wheelchair - in that case, if the ball returns towards the net after hitting the other side then it IS a let, regardless of whether it touches the net again or not.