Shape tween - rectangle turns into triangle

I’m trying to do a shape tween I’ve done many times before, but I’m having a problem. It’s a simple rectangle that I want to shape tween into a wider rectangle. It has a gradient fill. When I shape tween it, on the second frame the rectangle turns into a triangle and stays that way until the last frame, where it returns to a rectangle. If I add shape hints, it warps into an add shape. I’ve never had to do this before. I just want the rectangle to grow evenly. What might be happening? Thanks.