As mentioned there are weights also known to some as sliders that dictate the position of the clutch. These will move at a certain RPM due to centrifugal force. I have a Yamaha T-Max maxi scooter and I can feel when these weights move. Possible that one or more of these were sticking or partially hanging up during this transition. That would cause what you are describing. I think I would put some more miles on it and keep tabs at what speed/RPM this occurs and if it goes away with some more miles put on the bike.