This happens because your Position X does not provide enough room for the new Width–it would cause the right edge of the rectangle to go outside of the video frame, which is not permitted. Use the on-video control and notice how it does not allow any part of the rectangle to go outside of the frame. This is intentional, by design.
I reproduce it if I do not change Position, but I first change it to 0, now it lets me input 640
From the Documentation - English
you cannot push any part of the rectangle outside of the frame.