I almost want to say that this can be explained away somehow. Maybe I just missed something but did we ever actually see them fire a beam in the future? It seemed like it jumped right from them trying to detect the anomaly to them being attacked by ships. Maybe the anomaly was created in the state that it was in when the future enterprise went back to it, and we just didn't actually see this happen?
If the anomaly was generated to it's visible state, then it could shrink into nothingness from the point of the generation.
But again, maybe I missed a critical detail in the script.
EDIT: Okay, I admit that I'm confused. It was a long ep. What did they do to generate or maintain the anomaly in the future?
(After a visit to Memory Alpha to fact check)
Past and Present report anomalies in the Devron system, so Picard figures that's a clue, and he needs to know more about it. In the future, the Medical Ship can't find any trace of an anomaly.
Future Data suggests that they can look harder using a modified Inverse Tachyon Beam. Picard orders him to do it. Picard then orders Past and Present Datas to hit their anomalies with the same.
The Medical Ship is attacked and Riker beams the crew off and hastily retreats with the Future Enterprise, refusing all requests to investigate.
Past Data suggests they look inside the anomaly using a not-yet-available tomographic imaging sensor. Present Data says they have one. They look inside. They see three inverse tachyon beams converging on the center of the anomaly, and they're identical, which can only mean that all three beams came from the Enterprise.
Picard explains the situation to Future Data, who says that the tachyon beams probably created the anomaly (a paradox created by Q), and they should convince Riker to turn around so they can "watch it form", and at minimum the other two beams should be shut down.
Future Enterprise never used a tachyon beam. The only explanation I've seen for it is that Future Enterprise got a tachyon generator upgrade at some point and donated their old used one to Beverly's ship.