I think that the earnings multiplier remains active long-term. That's simply my impression, and why I keep a bunch of boosted stories pinned, as they are more likely to see repeat traffic long after the event.

The boost effect in the algo is time-limited, but I think the earnings coefficient is for keeps.

I had a similar experience just when FoM started. An FoM read an old boosted piece, clapped, commented and followed, and it registered at over $5.