So the candy timeline epilogue (the one that yiffy exists in) jade implied that merging with bec left jade as more of a… Genetic doner rather than genetic reciever. and that she talked to rose about being a surrogate

Now judging by the reactions, Kanaya seems to have know idea who yiffy is but rose likely does

My theory is that rose did go through with being the surrogate for jades child and their daughter is yiffy

read this with the theory in mind