Thanks for your inquiry. We have tested the scenario and managed to reproduce this issue. This issue is related to missing feature WORDSNET-721 (Improve table bookmarks). You will be notified via this forum thread once this feature is available. We apologize for your inconvenience.
If the BookmarkStart and BookmarkEnd nodes are inside a Cell node, you can extract the nodes of a cell using CompositeNode.GetChildNodes and import them into empty document using NodeImporter.ImportNode. Hope this helps you.