Which issue are you talking about? As far as I can determine, there are three reported issues that meet your description.
One is lurching at speed. This can be caused by a malfunctioning engine clutch, usually because the small volume of hydraulic fluid is contaminated. This is why this car has an unusual maintenance item of flushing and replacing the clutch fluid at 22,500 miles. Reports on forums are not necessarily followed through, but some high mileage owners in countries where the dealers didn't know about the clutch service experienced this issue and we presume that was hydraulic fluid related.
Another cause can be malfunctioning (defective) injectors. This is particularly hard to track down as it doesn't throw error codes.
Finally, delayed acceleration from a full stop is often described as lurching. Using sport mode when a fast start is desired (turning into or crossing traffic) solves that. When I first go the car, this was a bit disconcerting but rather than start mode I made my move a half second early. After some 14,000 miles, I have learned about some brake behavior affecting this. If you press on the brake hard during or after a stop, you will experience this slight launch display. But if you brake lightly, or release the brake and creep a few inches and apply brakes lightly, it will now launch normally.
If you experience lurching at speed during a test drive, I'd pass on buying the car unless it gets definitively fixed first. There were a number of 2107 cars affected but I'm not sure why or if it was the clutch or some software issues. Haven't read about it for late 2017s (check the date your car was made) or 2018s on Niro or Ioniq forums.
2018 Kia LX HEV Metal Stream with Advanced Tech