Iíve seen this a few times but chalked it up to passives or talisman or whatnot, but I finally found an example so obvious it could not be anything but.

In the arena:

Opponent has two chars alive:

Sekmet with valour
Killer Prime with Tower/warlord

Sekmet is at half health. I crit her with Alexander Eddie for 148,000 damage, but instead of dying she goes to full health.

I later kill her with a few successive hits in the 10k range.