I don't think it sounds mad, and it isn't like he has regressed technically. It's just that his first two steps are quite a bit slower than they used to be. Bale was much the same, except for the first touch. When I say technically, I mean like being able to get out of small spaces when closed down by multiple players, feinting, trapping the ball and such, like Ronaldinho, Cristiano and those types, not the basics. So we might think of two different things, mind. But with the ball at his feet at full speed running at defenders, Son is technically great (as were Bale).
To me, it looks like Son is being asked to do things he's not very good at, while he's not doing much of the things he's one of the best at. Running into space, crossing (in my head, he's a more than decent crosser, much more than the decent crosser Davies, who's an awful crosser), and not least shooting.