Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.86
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
Also broken: 2.70a - 2.79b (didn't test with older versions)
Short description of error
When B-spline rendering and interpolate option of the add hair tool are enabled, the added hair get shorter as you drag the brush.
Exact steps for others to reproduce the error
Add a hair system with 1 particle
Turn on B-spline
Go to particle edit mode, choose add hair and turn on Interpolate
Click and drag on the mesh.