What is the problem you see as a bug? Border was turned on and set to white, and then there was a white border. Before it was turned on (start of video) there was no border, as expected. What's the bug?
The "bug" is white horizontal lines appearing at bottom and top of the frame (not the vertical border). It's clearer if you also encrease the feathering.
I think it is expected and desired behaviour if you use a different angle. Actually for a 0 degree angle I would rather use the Push Transition.