This behavior is expected by design - when the DropPosition is set to Inside the preview line is not displayed. By default the preview line presenter is hidden. In order to display the line when the position is Inside you can write custom code. Basically, you can get the Canvas element that represents the preview line and update it manually in the DragDropManager's DragOver event handler.
I have attached a sample project how you can implement this behavior.
I hope this helps.
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