Wanted to chime in and mention that the Juju Flurry buff is currently matching the behavior exhibited in the 1.12 reference client. As others in this thread have correctly guessed at, Juju Flurry behaves similarly to Seal of the Crusader in that it increases your attack speed but also lowers your DPS by the same amount.
We also did some digging into our bug database from the Original WoW era and found that we had in fact filed a bug for this behavior while the original game was current, but the issue was not actually fixed until the Burning Crusade pre-patch (patch 2.0.3). As this bug was present throughout Original WoW, we do not currently consider it to be a bug and is merely an aspect of the original game that was carried over into Classic.
We will update the Not-A-Bug list with this information shortly. Thank you!