It turns out that there is another requirement for the bug to manifest: "Ending Screen" must be unchecked for the slideshow set to use for Impromptu Slideshow. Revised steps to reproduce:
1) Slideshow module
2) Click on default template
3) Uncheck "Ending Screen"
4) Save as a user template "Bug"
5) Set template "Bug" to be the one used for Impromptu Slideshow
6) Develop Module with a folder selected with 174 images
7) Click on 4th image in filmstrip
Actual Result: The slideshow starts with the 3rd image in the filmstrip.
Expected Result: The slideshow will start with the 4th (selected) image in the filmstrip.
Can anyone else confirm?