On the Golden Deer route, I actually missed the opportunity to A rank Byleth x Petra, but I'm not sure when the cutoff point is, as I got her to A rank at the second-to-last chapter. I also missed Caspar x the Petra B-rank, and I think cutoff point would be the end of I believe Chapter 18. I'm not 100% sure, since I got to their B-rank afterwards, but it makes sense for that to be the cutoff point, as
I was aiming to get every Support I could in this playthrough (sans Rhea but whatevs), so it's just my luck that Petra was part of two missable ones for my route...
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