Not sure if this is intended behaviour but when disabling the canvas gameobject and re-activating it the Material UI Scaler resets back to Dont Change Scale
Unity 2017.1f3 using the newest Material UI 1.1.7
Found the issue in the MaterialUIScaler.cs lines 579~582
if (targetCanvasScaler != null && targetCanvasScaler.enabled && scalerMode != ScalerMode.DontChangeScale)
{
m_ScalerMode = ScalerMode.DontChangeScale;
}