I haven't seen any of the games, so I can't speak with any authority on this, but just to throw out a theory.
A lot of times this happens with raw players. When they first step on the floor, they put up good numbers, but really do it outside the system, and may not even be a net positive for the team.
But then as they get integrated in the system, start learning the right things to do, those numbers start dropping as the coaches start holding them accountable for rotations, and floor spacing, and things like that, and they also have to think a lot more, rather than just react, which slows them down.
This transition can really hurt their stats, even if they are actually making progress.