I try to draw a mesh with several faces.
Some of the faces are drawn some of them not.
When instantiating a mesh which is normally not drawn, with the index
The face front is determined by the winding order, not the face normal. The default winding order is counter-clockwise (CCW).
Have a look at the source of Geometry.computeFaceNormals(), and notice how the computed face normal is consistent with a CCW winding order.
Geometry.computeFaceNormals()
three.js r.58