Bug/Issue Can't get S rank in Act 7 Mission 2

I've been playing this mission for hours trying to get it in fewer turns. Got 19 the first couple times, then I managed 18, and I finally managed to do it in 17, and it still says A rank. I have S rank on every other mission so far trying to get 100% so it's a little frustrating to be so close to finishing and get stuck on this.