Thank you for the detailed description of the issue. It looks like a bug in your end, I sent it to the dev, and they will work on a fix, unfortunately there is no any ETA.
Hello David,
That issue should occur because both parent and subtask are selected when you enable the multiselect extension and add a subtask. So, when you drag the subtask, you are also dragging the parent task.
As a workaround, you can return false in the onBeforeTaskSelected event handler for the parent task: http://snippet.dhtmlx.com/5/47af7fb31
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