This is absolutely a bug. I had no such problems before CS5.5 What the OP described worked fine on CS4. I am using Windows 7 x64. I used to use this transition the same way. It’s easier and faster than creating a white solid and dissolving it. Yeah yeah, you can do it that way, but that isn’t what the op was asking. If you set the start or end points at 50, it should begin or end white. I will ask my friend if he’s has the same issue on his installation.