so the "potion" stone heals HP restores MP AND cures status ailments
...but when he was practically bathing in that stuff after eating demon meat for the first time it didn't cure the demon corruption effects, instead it only healed his body while the demon corruption spread and maybe helped him adapt to it
that either means (1) the cure ailments effect is weaker than the corruption effect of eating demon meat but the heal effect was stronger than the constant damage he was taking from said corruption OR (2) that it didn't view the corruption of his human flesh with demon energies as an ailment to be cured in the first place
#1 means it's simply not quite as OP as it has been made out to be
#2 means whoever or whatever caused that stone to exist either meant it for demons and it just happened to not harm human MC pre demon meat festival (cause purifying demons of their demonic power would be rather counter-productive in something meant to heal demons) or possibly, and far more interestingly, meant for it to specifically be used to aid humans attempting to absorb demon powers, especially since you then have to wonder how it ended up exactly where it needed to be to save MC's life