when you open tab, KATE is saving it in ~/.local/share/kate/stash/empty.katesession/Document 1. Then when you reopen KATE, editor should read this file and you have what you wrote in there. But when I reopen KATE, it's opening this file, but overwriting it and makes it empty, so you loose unsaved tabs. It was working before, you just have to open correct session, in which you created new tab.
Merging in with an existing report for this issue, thanks! *** This bug has been marked as a duplicate of bug 505135 ***