The function works. See this screen capture of the template being evaluated by the template tester.
My guesses are:
- the book wasn't updated on the Kobo so you are seeing the old value. Edit: Clearly not the case since we are seeing the 3.88. Sorry about the noise.
- the Kobo cannot display the 1/2 star character. What happens if you paste the 1/2 star character (⯨) (the character shows as a box on Windows Chrome) into the template like this (an image so the half-stars show)?

Edit2:
here is a link to the unicode page for half-star.