It seems that setting the start and end values of only ONE segment makes it do this throughout the Polyline. It may be a means to do it, but I'm trying to imagine a situation in which there would be any REASON to do it this way rather than by disabling linetype generation. It takes more work in the Properties box than the latter way; depending on settings and how much different you make a width entry, it displays the same value for starting and ending width in all segments but doesn't display a global width, which can only be confusing (for example, if a User wants linetype generation working again, and it says it's on, but it doesn't show that way, and the displayed widths of the segments don't tell them which one is off, if they even know that's the source of the issue); if the width of the Polyline actually represents something meaningful (for example, something that may be the basis for some calculated value), it could throw that off; it doesn't work with closed Polylines (in fact, it had bizarre results when I tried it); maybe a few other considerations. I'd be interested in any description of when it would make sense to do it that way.