I think your HSS might be suffering from a lack of coolant and some small excess of speed, as long as it's just 4140HT and not something you've hardened up special. I cut gears from 4140HT with regular HSS gear cutters and haven't had any real problem with it, but it is something that will point out the difference between a sharp cutter and a slightly dulled one.
It's the spline end that would bother me - got the specs on it? If not is the mating part available? If not it may be a fun time in the shop to make something to fit without something to test against.