Bug or HP Drain Rate?

[JesterBlaze]

https://imgur.com/a/C9g1Bc3

Note: The two Overlords on JesterBlaze side were previously Jokers if that matters.

At the time of this screenshot one of my Overlords was attacking a Mutant (who had 30% HP Drain Rate) and the Overlord next to him took 1343 dmg and was destroyed. I'm thinking this is a bug though I don't know how the Mutant's HP Drain Rate ability works.

[ecb_admin]

Please attach the image and paste it in post.
Did the Overlord activate it's ability?

[JesterBlaze]

Yes, the Overlord was under 800HP and had drawn HP from Treant as I recall.

[ecb_admin]

Then that's its ability?