Even though the value is not defined its value is undefined, which is treated as empty string in the context of the k-ng-model directive. On widget initialization, the directive will set the value of the widget to an empty string ("") (link to the code), which will force the widget to ignore the index option. Unfortunately, we will not be able to change this logic, for instance checking for undefined, because current implementation supports clearing the parent object.
Based on the given explanation, I would suggest you avoid combining index option with ng-model/k-ng-model directives. I believe that you can add an optionLabel to the widget and thus the widget will select it on load.
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