This is a bug that only happens in Open World, and it happens due to how Blizzard scales mobs with you in open world.
Basically, a mob have a “real” baseline hp, let’s say 100k, but the way you see this mob is actually 150k due to your ilvl, once you cast a drain life who deals 15k to it you will see the hp going from 150k to 135k(-15k), and would expect that drain life would transfer 75k of healing for you.
But in truth, the real hp of that mob went from 100k to 90k, so -10k and the healing you will receive will be 50k.
Values for the exemple aren’t exact and are meant to only convey the idea of how the system of open world works.
I REPEAT, this don’t happen at Dungeons, Raids or Delves, only in Open world.
Now, i do have my doubts this will be fixed, specially given Drain Life have been used in the past with some creativity based around damage scalling of low level mobs - https://www.youtube.com/watch?v=bsLc-v9KhXw
But also because its only happens in open world, in other PVE contents the scalling is done in a different way so the drain life who deals 15k will heal for 75k as intended.
Let add an exemple: https:/ /i.imgur.com/ZT2V5yT.png this is the HP of a Darkfuse Enforcer in my 665 ilvl’d warlock.
And this https:/ /i.imgur.com/eSOf1pz.png is the same Darkfuse Enforcer in my 619 ilvl’d Alt.
And this https:/ /i.imgur.com/jQGgBPP.png is my ilvl’d 554 Death Knight.
If my 554 Dk deals 100k damage to this mob, who’s mob hp is closer to the true hp of the mob, those 100k damage will look like an 180k hit on my warlock main details.
If any of you played Pandaria Remix you might be more familiarized this kind of scalling shenanigans that happen (lower level players being stronger/higher dps than higher level’d one if they had the same amount of stats)