This does not look like any light leak I've ever seen.

Since you are getting a dark band at the bottom of the photo, this means that portion of the negative is receiving less exposure than the rest of the film. That portion of the film is at the top of the camera during exposure. Since this band is pretty clearly defined, it looks like *something* is casting a shadow on the top portion of the film plane when you rotate the camera to a vertical position.

Could gravity be at work here? When you rotate the camera to a vertical position, is *something* moving down to cause this obstruction that is casting the shadow?

Another question: which direction are you rotating the camera when you go vertical? In other words, are you pulling the dark slide from the top, or the bottom? And, if you are pulling it from the top, are you removing it completely, or just pulling it up? If you are just pulling it up, perhaps gravity is pulling it back down ever so slightly, just enough to cast a shadow?