The function name is misleading and causes problems. Every other function with “InputKey” returns (or expects to receive in order to return proper results) values from 0.0f to number of keys so input key is NOT a 0.0f - 1.0f range. It is 0.5f between the 0 index point and 1 index point, then 1.5f between the 1 index point and 2 index point etc. For a spline with 4 points total, the input key is a range from 0.0f - 3.0f.
My fix was to set the duration to the number of points (keeping in mind closed loops). This makes Time and InputKey functions equivalent, but that is not ok for every use case. The change required to make it less confusing is really small, literally modify the existing function to return input key as used in other functions and add GetTimeAtDistanceAlongSpline which does exactly what the current GetInputKeyAtDistanceAlongSpline does. That avoids rename, which may be painful and change a lot of files.
Do you not see the issue? Does it need to be explained in a better detail? Pull request isn’t really an option, you rarely merge them into the engine.