Stronger bump would always intensify this effect. I don't know what's the magic behind this. Ondra could surely explain the technical reason.
Everything with geometry normal facing away from sun will NOT get any lighting because shadow ray would be generated under the surface, travelling through the cylinder object and hitting its backface when resurfacing.
Everything with geometry normal facing towards sun can get lighting. BUT the amount of lighting is dependent solely on shading normal. If it would be dependent on geometry normal, the bump map would have no effect, because it can only modify shading normals.
So: lighting amount depends only on shading normal. Geometry normal only has on/off effect. So there is sharp jump where geometry normal cuts off all lighting. Without the bump mapping, shading and geometry normals are almost identical, so the "amount" and "on/off" effect work together as they are supposed to.
Well, that's a great technical description of who it works. It can be good to understand, but it cant be an answer to a problem. Don't be afraid for this, but one thing is to understand who the software works, one other thing is to work with the software ;)
I can be happy to have your technical description, but if it does not solve the problem I think we can't name it as a "Resolved bug".
I have had some discussion (very interesting technical discussion) with Master Zap about some Mental Ray artifacts, he always have given me amazing technical reason for this, but no practical solutions. One of the reason why I choose Corona is about it's way to solve a lot of situations in a very easy and "natural" way. So, please, with a lot of respect for you, don't give me a technical reason for a defect, give me a solution ;)
Btw, without bump it works good. With bump it works good in surface with no bump effect, but it works wrong (sharp) in bump effect, as you can easily see it on the bottom of the cylinder. If it is known limitation, I'll wait for a solution. If it's me, happy if you will correct and teach me the right way ;)
best regards,
alessandro
Edit: sorry to have this late to give this answer, I've lost your one...