Enable Mesh Distance Field in Project settings
In Hair Material the Tangent input value directly Changes the Normal of the hairs; In WorldNormal View mode you can see actually The World Normal is exactly same as Tangent direction which is not right. As I understand Tangent is supposed to define the direction the highlight stretches, but not Normal.
Enable DFAO, in DFAO visualization mode, it looks dirty and ugly. It's partly because of the Mask blend mode with dither, but seems the major problem is DFAO sampling respect the incorrect Normal of Hair.
Disconnect Tangent input in material or use vector (0.0.1) as default value. DFAO looks right then.
Here's images show DFAO and WorldNormal between Custom Tangent is feeded and not
PS:SSAO doesn't respect Normal(Tengent) of Hair Shading, which is OK for practice
PPS: The custom Tangent could be any vector other than 0.0.1; The Tweenblaster character in this case is (0.-1.-0.3~0.3)
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-88361 in the post.