My guess was correct. I needed to sit back (Block) for a few rounds to let the DK regain consciousness to use her healing for full HP again.
But with huge bleeds stacked up, she was easy to take down to zero again, this time for good.
Not sure if I'd call this a bug, but it's weird. One lesson is that the combination of "Icy" and "Sharp" on a weapon is pretty handy against the DK.