The Contax 35mm was designed and made for a 645 film camera. It will give the field of view that you would expect from a 35mm lens on a 645 camera (or even a 6x6 camera).
If you use it to expose a 30x45mm area, you are not using the full 645 (6x6) area and therefore, rather unsurprisingly, the field of view will not be the same. It will in fact be similar to a 43mm lens on a 645 camera. The point being, do not expect your Contax 35mm to give you the same field of view on a Leica S2, as it does on your Contax 645.