I am also experiencing this bug. I gave my first potion to Mabel without getting a reward from her.

I did this because only one character received XP when I persuaded her to give me a reward versus all party members getting XP when I chose not to extort a reward from her.

I later learned how to make the potion and came back to give it to Geraldine. The dialogue indicates I can feed it to her but she does not transform and the potion stays in my inventory.