I agree it should be in the main panel. Although I have seen where the main breaker in a panel looks like a normal breaker but it is actually the feed. That probably would not make a GFCI breaker work correctly if that was attempted though. who know maybe they put a main breaker on both ends. It is all a mystery until we actually get to see a pictureBreaker for feed to subpanel should be in main panel.
We have to see what this subpanel looks like.