Neil and i talked on discord, sadly this is an issue that always was and always will be there (Even HS1, even default models)
This post is to anyone else thats interested, not at you
NealG1982
There are ways to hide the line somewhat, but you can never actually get rid of it. Its where 2 meshes meet and you cant avoid that sadly. It becomes more visible when head scaling is used, & becomes more of a problem
Tips - to hide the line
In material editor
Adjusting subsurfacealbedoopacity & subsurfacealbedosaturation can make it less noticable
You can also adjust in creator Bodytab neck width, done properly this works very well. Especially with the above mat editor tips
A few other suggestions Tattoos in body paint on the neck enlarged and matching the skin/gloss and texture. Try different paint types for best results, the faded circle as example can work well.
In body tab
Changing the tan can sometimes help as well (example freckles), this one is weird and results may vary
Selecting a longer hair style can also hide it