I've inspected the code snippets provided and as it appears you are grouping your items based on some attributes. The important part here is that you are adding the transferred item on a specific index in the items collection of the destination ListBox and it shouldn't be show at the bottom of the list as you have described.
In order to troubleshoot this issue, please open a support ticket and provide a runnable sample page demonstrating the issue so I could debug it locally.
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