While they did mention Sherlock's first case (Carl Powers), they also kinda did go back on that in TFP by calling Eurus' Musgrave puzzle S's first case. Which makes sense because S would have been just a child back then. Both 'first cases' (Carl Powers&Musgrave) were not solved by S at the time. He only solved them later. What if the show tells us about his actual first case, which S did manage to solve and that one's connected to an actual Victor Trevor and is significant for who S is?