I've had that happen to me, too, though it was at the end of stage three.

But yes, Vanaduke is a horrible fight. We already knew that. This makes at least three completely independent ways that the fight can be broken and uncompletable. And that's in addition to a number of other bugs with the battle that don't make it completely impossible. Even if there were no bugs at all, it's a horribly designed fight that still wouldn't be any good.