@Melissa
Oops... Maybe GPU issue? Driver up to date? 

Toystorylab on Vimeo : 75% of original size (was 671x19) - Click to enlarge    Crassitudes (my "Alter Ego") on Youtube: 75% of original size (was 671x19) - Click to enlarge

None of the above afaik. 



0

Is this happening with all hair cards ?
Raytracing on or off?

It's happening with hair cards from the Hair Builder pack. Happens with Raytracing and/or Lumen. And, I have tried with both anti alias on/off. Same result.

I just licensed a Blender addon for particle hair. Will try an Alembic export to make a groom in UE5. 

Using the latest studio driver from Nvidia.

The good news is that it works for you. So there is hope. It does make me question whether or not there is a setting in UE that has to be enabled, something in the editor or project settings? Not sure what that could be, however.

Yes, that is exactly what is happening to my renders/viewport. Makes absolutely now sense and it's baffling.

I used an addon in Blender called Facial Hair Tool; modified the particles to taste and exported as an Alembic file. Imported the Alembic into UE5 and it appears to have rendered correctly, or at least as expected.

 75% of original size (was 671x19) - Click to enlarge

I am having the same issue :@ ANY ONE FOUND THE CAUSE AND SOLUTION?
75% of original size (was 671x19) - Click to enlarge

My current understanding is that the CC Unreal hair shader needs an accumulation buffer (data from the previously rendered frame) to properly render out the hair opacity.
UE5 editor uses by default TSR anti-aliasing with such an accumulation buffer so the hair looks ok in the UE5 editor.

If you use Movie Render Queue then make sure to use Temporal Super-Resolution (TSR) Anti-aliasing setting, otherwise you will see these dithering artefacts.
Spatial sampling or temporal sampling will not help you here.
You also need to render at 30fps or higher otherwise the accumulation buffer will give you a lot of ghosting on fast motions

The hair LWHQ MaterialInstance has also Alpha Multiplier parameter which can be used to increase the hair opacity.

Most hairstyles I tried look bad (too bright) under Lumen.
Most hairstyles I tried turn fully black when using HDRIBackdrop IBL lighting with raytraced shadows.