WIP Invoice Credit Note with 'Return original costs to WIP' selected has not returned costs to WIP

On duplicating a WIP Invoice within a Project you are able to select an option to Return original costs to WIP.
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On selecting this toggle the duplicated Invoice Type (most commonly a Credit Note) will, when posted, return the original lines from the WIP Invoice into the WIP Invoice Selection.
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In order to ensure that the lines are returned to WIP you must ensure that within the new Credit Note, none of the existing lines are deleted or new lines added. They both must be a like for like number of lines.
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It is permitted to edit the lines, such as text detail, etc.