Due to a leak, my bathroom ceiling fell. When cleaning up I expected to find some kind of solid material,but, the material looks like a thin painted plastic sheet that the insulation rested on. Is this normal ?

Yes.. that was what is called the vapor barrier. There SHOULD be one... otherwise moisture from inside the house in winter would migrate into and through the insulation, condensing and making it wet and even freezing. You want NEITHER situation to occur. It just has to be a barrier that moisture will not pass through. Doesn't need to be thick at all.