Slope of spillway can also be towards the pool (gaining elevation). Typically they are installed in the same elevation as the original unless advised otherwise.
That’s a lot of Shotcrete (thickness) giving off heat during curing.
Only way to exactly know the extent of the crack and if their is a underlying cause will require exposing the area and finding the depth.
That is how most plaster companies apply glass during the application.
It is an art of applying in a uniform manner.
This may not have been the case on your spa.