The buff persists across stat increases.
You don't see it in the level-up-screen, but it's applied afterwards.
It's easy to test:
Make a save, Withers, eat the hair, level up.
Great to know, thanks! The hag dies tomorrow as scheduled!
Or, rather, almost dies, but yeah.
Is there another +1 buff in the game? Because 20 and 21 are effectively the same.
Shar's mirror of loss has some potential increases as mentioned here https://bg3.wiki/wiki/Permanent_Bonuses
Also the potion Araj Oblodra makes after Astarion bites her gives any character a permanent +2 in strength. Useful for martial characters.
I actually used it to increase two stats to an even number. Uneven stats don't give you any bonuses.
Every point of strength increases carry weight. Every point of charisma decreases shop costs. Not sure about the other stats but those two aren’t entirely useless at an odd number.
I think jump distance benefits from odd strength to too, though I'm not 100% sure on that.
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