When using the metronome feature with vibration, the cadence reading occasionally drops to half the actual value, which skews the data in the end.
I do not experience the same, but in your place I would just use the acoustic beep in such case. Perhaps attaching the wrist band a bit tighter, or running on a harder surface (or using different shoes) could help too, but apparently the vibration adds a too high noise to the accelerometer. Of course, you could also contact the Support, and report it to them, but I do not quite believe they would help you much.