This is how all self-balancers work, once you hit max voltage, further acceleration, which is necessary for the automatic balancing to work, is no longer possible. Once you are pulling the voltage that the battery is at, that's it, there's no going faster, meaning the second you hit max speed, the self-balancing is also gone.

OneWheels being particularly weak performance-wise probably makes them worse than most, tho. Most self-balancers have plenty of extra speed to spare above their advertised top speed.