It copies the named ranges in the worksheet but it doesn't copy the ones with workbook scope. The reason is that ClosedXML can't guarantee that the referenced sheets will be available after the copy. If you know they will be then you have to copy the named ranges yourself. Excel deals with this by referencing the actual source file the worksheet was copied from.
See our other thread: https://closedxml.codeplex.com/discussions/549543
See our other thread: https://closedxml.codeplex.com/discussions/549543