As its clearly a issue with the skylight interaction with hair strands, I tried looking into some cvars and found potential solutions.
Default results in dark areas with bright skylight shining in through windows/doors etc.:
Better lighting in dark areas with bright skylight, but grainy/noisy:
r.HairStrands.SkyLighting.IntegrationType 1
noise can be compensated by using higher values for
r.HairStrands.SkyLighting.SampleCount
. . . at a high performance cost.
Disable skylight contribution to the hair strands:
r.HairStrands.SkyLighting 0
My prefered setting for good visuals is
r.HairStrands.SkyLighting.IntegrationType 1
r.HairStrands.SkyLighting.SampleCount 32 OR 64 OR 128 - depending on what you can afford
not great for production, but works well for standalone things