Evernight is supposed to do true damage to a random enemy.
With that said, I’ve been noticing it to skip this true damage application at times. If it helps, there was only one enemy left (a dryad). I noticed this bug to happen on several occasions now.
I’m not sure if there any link, but the enemy usually has been poisoned by Evernight several moves prior.
However, it seems that they sometimes take the true damage, while sometimes they don’t. I can’t pinpoint the pattern.