For me, whenever I defeat her normally (tip: toggle on nonlethal attacks with your martials, then it is easy to get her to <20 HP without killing), I get the dialogue that she gives me the hair, including the cutscene of her ripping her hair out, but the hair never appears and I do not get the choice of which stat I want. Instead she just disappears.
I have tried this many, many times, and got this result consistently (party: Bard PC, Laezel, Wyll, Gale, no mods, Stadia)
However, the solution I have found is to ask for both Mayrina and the power. That dialogue branch is not bugged and you get the hair as normal.
You need a persuasion check of 20 for that, but with a bard and a couple inspiration points (always take the friends cantrip and/ or guidance from the necklace too!) it was easy.
I did this twice because I get a bug and had to use my last qucksave, which was quite a while back, and again I could not do it normally, and again, getting both the girl and the power worked