I have been having this theory for a while now. It really fits Sunmi's concept, and is a perfect follow-up to Gashina. I am pretty sure that the song was made for Sunmi, then it didn't work out, so they released Heroine instead.
Everyone seems to think it is... maybe it's possibly something Teddy had in mind for Sunmi, but eventually ended up giving to Jennie? So technically not exactly a 'reject'