IIRC it’s because the contact shadows are screen-space calculated, so in essence the engine can’t know what’s behind another object. Same goes for AO. (someone correct me if I’m wrong…)
I was also quite bummed to learn about this limitation, as it makes many animations really twitchy to look at. There is the “overscan” -option in the film settings, which helps with the borders of the image, but the problem remains in these other cases.
I’d be very interested to know any other methods to achieve global AO without this limitation, but I’m not sure if it’s possible yet