The sine wave is moving with Get Game Time in Seconds which is the total game time.
This means that the moment you pass the branch test you are immediately sampling a point somewhere on the sine wave (depending on the game time) and it will most probably be far from 0, hence the jump.
If I were you I would probably add a short-timed blend from 0 to the sine result and blend back to 0 when the branch fails (when you are no longer moving).