This relates to my previous post below. I've now narrowed down the problem more and feel confident reporting it as a bug.
Steps to reproduce in SMS 13.0.8:
Make sure a scatter set that is appropriate to use for a size function is present.
In the Map module, create an arc that isn't a straight line, positioned such that some extrapolation, as well as interpolation, would be required to get estimate a value from the scatter set for some regions of the line.
Select the arc in the Map module, right-click, choose Redistribute Vertices.
Under "Arc redistribution", specify a size function and in Options, choose the scatter set in question. Choose linear interpolation, and open the next Options dialog (which is titled "Interpolate - Linear").
Leave "Truncate values" turned off. Under "Extrapolation", select "Constant" and enter a positive number in the "Value" text box. Remember this value.
Switch the extrapolation method to Inverse Distance Weighted.
Press OK through all the dialogs, so that the redistribution operation is performed.
Inspect the parts of the arc that required extrapolation, and note that inverse distance weighted extrapolation has not been done as requested - instead, the spacing between vertices is exactly the number of metres that you entered in the "Value" box.
Hopefully others can reproduce, and it isn't just something that happens with my dataset....
If I can usefully provide any further info, just ask.