I was going to say I'm sure you're overlooking something, but a search turns up quite a few people complaining about this over the years. One person said it was possible to create a dummy character and have them complete the quest and then steal the scroll from the dummy character (like what you did with your friend, except take the scroll and screw the dummy character). But that requires quite a bit of effort. Especially if it's the Nightmare or Hell quest that bugged for you. And it sounds like you're not sure which difficulty it is in the first place. If I was really attached to the character, I suppose I'd try the dummy thing.
Actually, you may have solved the problem for me. I think this is a fast and easy check on it being normal Anya and Radamant. I'm going to explore this. Something I remembered is that I had a scroll in my inventory that was red that I couldn't use, and I couldn't figure out why. I thought I had used mine and accidentally picked up somebody else's.
I bet it was the normal Anya I missed because I didn't use the inventory during a normal game or something, tried using it later and was like oop I messed somebody else up... only to realize I messed ME up lol