Question Generating an animator via code, cant set the sync layer correctly.

Hi I am trying to generate an animator via code, but the part where i say which sync layer it should point to is not working as expected.

var controller =
AnimatorController.CreateAnimatorControllerAtPath("Assets/TEMP/GeneratedController.controller");
controller.AddLayer("Layer 1");
controller.AddLayer("Layer 2");
controller.layers[1].syncedLayerIndex = 1;


According to the reference this should cause the layer to act as a sync layer,
https://docs.unity3d.com/ScriptReference/Animations.AnimatorControllerLayer-syncedLayerIndex.html

This is not working,
Am I missing something?