I have never seen that - seriously. I have stayed on my personal device maybe ? twice in all the times I have ridden a Disney bus, and Every. Single. Time I don't sit on the device, the driver has used all three ties downs to secure my (now riderless) device. Every. Single. Time.
If you are experiencing anything different, I would get the drivers name and bus number and contact the Transportation department ASAP. That driver needs to be retrained.