I see now what you mean by Input Key causing problems. Here are two instances of the same Blueprint, using my answer to this question to evenly-space four spline meshes across three spline segments. The only difference is that the top one has Input Key values of 0, 1, 2, 3 for the four spline points, while the bottom one has the first Input Key set to 0.5.
If you set each Input Key value to the index of the spline point, Get Location At Distance Along Spline seems to work correctly. If not, it’s wacky. I can’t say for sure that it’s a bug, because maybe this is what “Input Key” is designed for: some way to weight spline points as percentages along the line (or something).
