In the episode All Good Things in the future time period the Pasteur fires a tachyon beam which originates the anomaly. It is an Anti-time anamoly so grows backwards in time and not forward. So when the Enterprise commanded by Admiral Riker returns to the spot there should not be any anomaly visible.
The Riker Enterprise is ahead in time of the creation of the anomaly and the anomaly only grows backwards in time so once they created it they should never see it unless they travel backwards in time which they don't.
~~~~ Unless Q hid it as part of the test.