It could be debatable if the description didn't state neatly the contrary. Your suggestion is not a suggestion (what do you actually suggest?), and if the idea was to change the kata design, it won't be done since it would invalidate more than 50,000 submitted solutions.

This may be debatable, but if the ball is starting at the same height as the window the result should be 1 and not -1

edit: especially since there's a possibility that the resulting height after a bounce is the same as the height of the windows, thus falsifying the result

As a Mechanics Enthusiast I have knowledge of Bouncing Ball Problem, There can be posibility : bounceHeight = window, at that time ball can be seen only once. We can add this constraint to make it interesting.