You're not giving enough information for anyone to help really.

However, I'll take a stab because I'm pretty sure it's because your acceleration is too low and you are using constant velocity G64 so you are getting corner rounding due to fact Mach is trying to maintain the Constant velocity command but due to physics there's no way it can cut that tight a radius so to get around the corner it cuts across the grass so to speak.

The cure is to up the acceleration in motor tuning or change to exact stop mode, but this could give you a jerky movement dependant on how the code is generated.! Playing with constant velocity angle settings will also help.

To get the higher acceleration you may need to lower velocity in motor tuning otherwise you could get missed steps. You can't have both high.