Haxe CreateInstance and Properties

I want to deserialize an object with properties. The object is well constructed but setters/getters are not correctly instancied if I don't type explicitly my object.

Is it an intended behavior or a bug ?

Minimal example:

package models;
class TestClass
{
    public var test(default, set): String;
    public function set_test(myVar) {
        trace("Set " + myVar);
        return test = myVar;
    }
}

class Main
{
    public function new() 
    {
       var typedTest: TestClass = Type.createInstance(Type.resolveClass("models.TestClass"), []);
       var untypedTest = Type.createInstance(Type.resolveClass("models.TestClass"), []);
       trace(Type.getClassName(Type.getClass(typedTest)));   //"models.TestClass"
       trace(Type.getClassName(Type.getClass(untypedTest )));  //"models.TestClass"
       typedTest.test = "12";  // "Set 12"
       untypedTest.test = "15"; //nothing happens here
       Reflect.setProperty(untypedTest, "test", "18"); // "Set 18"
    }
}

I'm kinda confused about this one.