Interesting observation!The corner that it leaks in is actually fairly well guarded from rain with the roof coming over the cover a good amount.
The leak is most likely just because water is being sent up under that part of the roof and it is probably adding water to the space the cover is stored faster than it can drain out.
I think it's still possible to have some leakage in that corner as well as the driver side corner during very heavy rain. But overall I think the Cybertruck bed seems to be well sealed where it matters.
I also think future car wash tests, at least automatic car wash tests, will also show leaks.
I would like to see a test with someone positioned above the Cybertruck, using a hose or pressure washer from a high angle.