This was/is very common on new 990's. Mine use to do that, but like most others, the problem just went away once the bike had some miles on it. I think it has something to do with the clutch pressure switch signaling the ECU to add fuel when the clutch is pulled in. I believe for emissions reasons, they cut fuel during engine braking, but when you suddenly pull in the clutch the FI system has to add fuel to keep the engine above tickover. Make sure you fully engage the clutch when downshifting. Also, the 15 minute idle calibration can help too. Maybe someone else can say exactly, but I think like fine wine, the clutch switch gets better with age.