Gradient sometimes zero (which prevents a gradient decent optimizer from decenting)

I have experienced many cases where I can extract a path with the [IterateNeighborhoodOptimizer](https://itk.org/Doxygen/html/classitk_1_1IterateNeighborhoodOptimizer.html) but where the [GradientDescentOptimizer](https://itk.org/Doxygen/html/classitk_1_1GradientDescentOptimizer.html) does not start to decent even though the speed function is continuous (based on a binary segmentation), apparently because the initial local gradient is calculated to be zero (even though start- and end-point, speed function etc are kept the same).
I have a vague feeling that this thresholding:
https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/6ae35b9d044ff2be22ce42776c64258be9252313/include/itkSingleImageCostFunction.hxx#L145-L152
to a hard coded value of `15.0`:
https://github.com/InsightSoftwareConsortium/ITKMinimalPathExtraction/blob/6ae35b9d044ff2be22ce42776c64258be9252313/include/itkSingleImageCostFunction.hxx#L140
could be the culprit.

Another possible reason I could imagine is that the floating point precision might not suffice under some circumstances to calculate a non-zero gradient even if there is no extrema in the cost function at the current optimizer position.

I use this CLI for the testing:
https://github.com/romangrothausmann/ITK-CLIs/blob/bfb1312142d505cacd6770e4d5acc23475290c8f/min-path_seg.cxx