That's from a steam report I got. In general, if you take [skill that gives +X mana] you can never go below [x] maximum mana.
I think this may be working as intended, and that it's more a case of your mod conflicting with existing game balance. Near as I can tell, there isn't really a situation in normal gameplay where you'd have so many mana debuffs that it would be 0 if you had no bonuses, with the exception of characters who really aren't focusing on mana in the first place and are likely wearing a lot of -mana gear. Heck, in normal gameplay it may even be considered a benefit of +MP gear, in that you always have enough for some reserves.
It's not mana gear, it's only mana skills. This is kind of an issue for both a skill which I have made (ok, yeah, big woop) but also from -mana that various monsters throw around like candy.
