This may or may not be a bug, but it's something interesting I noticed just now that I want to share.
I was running PvE levels solo with Astrid, and of course the enemies couldn't get past her 'Just a Scratch' passive. However, I also noticed that this time around, they were dealing 0 damage instead of the usual 1 damage. So, I still had my 'Use 2 potions' quest to do, and tried to use a Health potion on my supposedly full-health Astrid - but to my surprise - It worked! It let me use a Health potion on Astrid, despite the fact she should never have taken any damage at all.
Conclusion; 0 damage isn't really 0 damage, and damage probably goes into a few decimal points, actually. So in theory, a Hero with 4,000 Health could appear to take 3,999 damage, but still die. Because of decimal points. I dunno. Just wanted to share this neat discovery.