This only happens when
1) Hair Width is very small. e.g. 0.01 in the example
2) Light has raytraced shadow and Samples Per Pixel has a value > 1. e.g. 8 in the example
The problem seems to be related with rasterization. r.HairStrands.RasterizationScale 0 workarounds ** it. If this is by design, it would be very helpful to reflect the parameter in the UI.
Open attached project to reproduce
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-94065 in the post.