Added a new bug:
http://bugs.calibre-ebook.com/ticket/7238
When enabling this option and and converting to MOBI (not sure if other formats are impacted), the tags metadata line is repeated -- but, on the second line, all commas are replaced with "ttt".
Checking the metadata.opf file shows no duplicate tags or "ttt" entries. See ticket page for examples.
Anyone else had this problem?