So, when I'm testing for implementation correctness, rather than balance testing, I am a big cheater. I usually use the in-game debug tools to give myself a bunch of Green Essences that I chug like water as needed while I'm testing to see if whatever new ability I'm working on actually works the way it's supposed to.
I usually delete test characters fairly frequently, but for some reason I'd kept this one around awhile... so long that, in fact, his Vitality had gotten to where it was if he had spent a bit more than 6 points/level on it (listed bonus was +133%.)
Then, I noticed a funny thing happened... enemies no longer wanted to attack him.
I don't mean that they preferred other targets, but would take him if they could get him. I mean they Would Not Attack Him. Even if he was vulnerable to the attack type!
It turns out there was a rather horrifying bug in one of my AI evaluation functions.
One of the lines of code in there is basically this:
vulnerabilityValue = 200 - (targetEffectiveHealthPercentage)
targetEffectiveHealthPercentage is the target's health percentage, but modified by any bonuses they have to MaxHP, so that it properly reflects how vulnerable they actually are. Of course... if you have a bonus to MaxHP that is > 100... vulnerabilityValue... actually... becomes...negative.
Negative, to Demon AI, means "this is actually going to help your target, don't do it!" Which is not really an accurate assessment of the situation.
Anyway, it's fixed in the next build.