I'm using Table t1 = sh.createTable(); but I found that the TargetURI of the newly created table seems to be wrong, in fact is set to "/tables/table4.xml" instead of "xl//tables/table4.xml" as all the other tables are in the file. Even inside the XLSX file's structure this seems to be wrong, all the other tables are located under "xl/tables" but my new table definition is located under "/tables".
Fixed in r1092281. I've also renamed xssf.model.Table to xssf.usermodel.XSSFTable as mentioned on the list, as it seems the usermodel is a better home for it given what it does.