Hi all,
Good news! Thanks to all of you that have sent me your save files, I've managed to track down and fix the issue many of you were having with world titles not being defendable after winning them.
As a further plus, you won't need to restart your saves or continue with an undefendable title - if you've encountered this problem, I've added code to vacate the title so that you can win it again and go from there. Sorry for taking so long, the problem itself was simple but I had trouble tracking it down.