DI usually works, but sometimes my toons just bleed to death

[Noren]

This has been hit or miss behavior, and I'm not sure exactly how to reproduce it yet, but here's what's happening:

DI goes off and heals a low amount (4500hp in this case), and my toon lives through the death scenario but is unconscious and bleeding to death on the ground. Heals cannot save this person. WoA regeneration does nothing. Minutes go by, and finally that toon dies. Here's a pic. It has happened multiple times, but usually as a result of a save from Creeping Doom. And it doesn't happen every time (frustrating for finding the source).

This particular toon (bard) had 56.6k-ish hp if that has any bearing on how long it took to bleed out / the DI heal calculation.