I don't know if that is a glitch or a bug.
its by design
How so? If Shadowheart was knocked out, I should be able to revive her. However, dead is dead. I should be able to revive unless I have a scroll in my bag. It's cool because the game is seriously fun!
The short answer is plot i.e, Larian made it so whatever you do Shadowheart must appear at a certain point in the story... They made these changes in a panic just before release and this 'fix' is the result
Play the game and you will understand later
