curves

Where is the anomaly? What do you mean by "end smoothly"? Every curve in your plot ends smoothly to my eyes.

The software decides when to stop drawing a curve by the power criteria I mentioned. Some curves might end earlier and some might end later due to the calculation is done at each HV step. You can reduce the HV1 step (eg. to 10V) and check if that's the result you want. Note doing so will make the testing time longer.