I have this same problem. I have two canvases in world space. The second canvas has a button that cannot be clicked when running under 4.6.1. It was working in 4.6.0. I have installed 4.6.1p2, but it's still not working. I tried playing with the setting in both canvases and changing the z order and the order in the layer, but nothing seems to change. Am I missing something? What do I need to do to make this work?