Ahh thank you! Most recently I recall playing...
spoiler for Serin's route
... when the MC and Serin are kissing in the MC's room just before his coronation. It starts the moment I click "spur him on" I think, Serin stays in the room and they are about to make some love and there's a CG of them kissing on the bed. The track starts with a piano, after some time I think that's cello that kicks in? It's a very mysterious, magical and whimsical tune. I love it, it gives me so many feelings.