Right, I guess CalculiX internally doing some treatment such as equal displacement constraint by Equation described for composite shell. But, I’m not really know what approach actually implemented.
An explicit modeling of layered solid element still better and faster using tie constraint as proposed or tied contact type. The last one is not sensitive to improper definition of master and slave surface of non conformal mesh.
Also, I did not repeat the discussion since it clear by many post replied. More concern is in composite shelll implementation in CalculiX by some screenshot above.