Came across this again:
I'll copy and paste my comment on Youtube (honestly this is meant to be a very positive thing, I don't intend to imply she is faking anything):
"Man it played out so perfectly too!! Created such an amazing moment with Fukuchan running and Sayu toughing it out, and resting until Suki da na Kimi ga where she danced. Sayu is such a smart person, if it weren't for her genuine reaction (the feels!) then I would think she planned it to make such a perfect moment for a one of a kind concert. It was really so symbolic tho."