Thanks for the provided scene. It's related to the interpolation of the base spline - disabling the Editable Spline > Optimize feature and raising the Interpolation Steps value to 32 seems to fix it. Optionally, adjusting the RC object under Style > Curve Steps option helps also. Attaching screen-shots.
Hope this helps you and if there will be anything more we can help you with, please let us know. Have a great day.