The shield does not always shield.

I noticed that sometimes I was getting hit for +400 and death shield was not healing.

Here is a pic showing one big shot not healing anything, usually the heal takes place before the damage. This happens a lot vs archers.

https://cdn.discordapp.com/attachments/579860908080234536/606230780091301909/unknown.png

 

It seems to me from your screenshot that it did in fact heal you, but did not take into consideration the critical hit multiplier, as if it only healed the amount if that hit had not been a crit!

 

crits go through death shield, I thought this was common knowledge and intended