iMac 27-in late 2015; 16GB; AMD Radeon R9 M395X 4GB
Broken: recent builds of 2.80 and 2.79 (feb 2018); 2.78c; 2.75a. Worked: none yet
When you open the attached .blend file,
The lower one has a vertex group for length, that covers the entire object,
whose weights go linearly from 1 to almost 0, and its effect on hair length
is not quite what one would expect, but close:
weights of 0.9 and 0.8 appear to have the same effect as 1.0,
and the shortest hairs are about 1/4 the length of the longest, as opposed to almost 0,
but there is, clearly, a linear dropoff from vertex weight 0.8 on downwards.
This I observe with interpolated child particles, no kink,
or with kink = Curl, Radial, Wave, or Braid.
Note: Kink = Spiral does produce a length dropoff that looks impeccable!
The upper object is a simplified scalp from my WIP here: https://sta.sh/21968qcn7oqe
It also has vertex groups for density, length, and kink, but here the discrepancy
between the expected hair lengths and what one sees is much greater.
In this case, even kink = Spiral fails to satisfy length expectations; moreover,
less than half the hairs receive a spiral, even when I’m not using any vertex group for kink.