A focal plane shutter has two curtains. One of them is spooled up and the other is blocking the film gate to keep light from reaching the film. When the shutter fires, the first curtain is released and it spools up on the other side of the film gate, opening it for the exposure. The second curtain is then released, the time delay depending on shutter speed selected, and it covers the film gate, the edge of it meeting up with the edge of the first curtain just past the other side of the film gate. The only way the shutter could be opening halfway would be a "capping" situation in which the first curtain is slowing down or stopping partway through its travel, with the second curtain catching up to it, and pushing it the rest of the way across the film gate. Unlikely if it's truly affecting half the image. Shutter capping almost always affects only a strip along an edge, no more than about 1/5th of the frame. If it caps at half the frame, the shutter almost always jams up and becomes entirely inoperative because half the frame is too far to push it the rest of the way across and force it to spool up. Again, it's possible, but IMO not that likely.

IMO, the most likely culprits are sync shutter speed, or the camera's sync timing is off. IOW, shutter speed is running faster than 1/100th for some reason when you have the flash on (which Jon mentions), or the camera is running at the correct X-sync shutter speed and the timing of the flash firing is off for some reason. Either one could have the flash firing when opening curtain is partly covering the film gate.

The IS-20 has a vertical focal plane shutter and the curtains in all the ones I've seen travel from top to bottom. The image on film inside the camera is upside down and reversed. From your description stating the upper half of the photo is "black" (badly under-exposed), this means the bottom half of the film gate is blocked, and hence my conclusion the opening curtain not clearing the film gate completely before the flash is fired.