As you can see :

Why it happend?

Try Sky.frac(1.0)

or use atmospheric sky with starLayer (which accepts cube map sky box)
and set atmospheric color alpha values to toggle transparency between atmospheric sky and star layer

X3llos used atmospheric sky and starLayer.
We prepared cube map with stars and we used function:
and effect is visible on the screen.
Later, we tested Sky.frac with different options and the effect was the same.
We also changed the options related to the alpha color and nothing has changed.

try using atmospheric horizon color alpha 0 or 1

Does not work. Can you create tutorial about 'how use stars'?