As an update on this, I created blank new 5.1 project, imported MetaHuman there and saw that the grooms are way smoother than in my 5.1 project (upgraded from 5.0).
Reference picture (Left - Blank 5.1 Smooth Hair | Right - Upgraded from 5.0 to 5.1 Messy Hair):
Try to compare all .ini files inside of the old and new project, like by TotalCommander integrated folder comparison tool. And especially file like DefaultEngine.ini
Well I remember already playing with it but nothing changed, now based on your comment I tried it again and it works (Copied the whole DefaultEngine.ini [/Script/Engine.RendererSettings] content from blank 5.1 and put to the other one).
After implementing all the rendersettings from a blank project .ini, UE forces me to revert them to the original since they are required by the MH.
For me, thin hairs are only visible as artifacts when being very close to the MH. (See below)
When I increase the Hair with or the scale at the root or tip, its overall visible but looks very ugly. Is something like a minimum object size to be displayed which differs from UE5 to 5.1?