skill description: If level up +30 more, if AP applied +20 more on top of MAX HP. When I add fresh AP into my main stat, it doesn't seem to be increasing my HP at all.
Oh, as usual Nexon's skill descriptors are poorly formulated and easy to misinterpret. The skill gives you a boost in HP gained when adding AP into HP, not AP into other stats. And of course, it adds extra HP upon level up as stated. You should max this skill. There's no bug with this skill as far as I know (but man, skill descriptions could use an upgrade...)