Hello @Mike_Woo ,
Lasso strokes in ZBrush now support Lazy Mouse, and this is enabled by default. That is the cause of the little red lead line.
However, this by itself should not cause what I’m seeing in that image. If you should draw the stroke out without touching any other modifier keys, it should draw out smoothly at the default settings.
Lazy Mouse does have a functionality where if you hold down Shift and then release it during a stroke, it will draw a straight line. Is it possible you are invoking this functionality?
If your Lazy Mouse settings have a high stepping (LazyStep) value, that would also produce this issue. The default is .25.
Lazy Mouse can be disabled in the stroke palette in Stroke> Lazy Mouse.
If that does not resolve the problem, this may indicate a problem someplace else. If using a tablet, you might want to unplug that tablet and try this with a conventional mouse to rule out any tablet side issues.
If the above does not help you identify what is going on, please contact Pixologic Support.
Good luck! 