Hmm none of these seem to have an effect. I have another very simple brush to try to find what causes the problem.
As you can see, my brush is a simple sharp white circle. But in 3DC, the alpha that displays on the cursors has some sort of "elevation" on it: lighter borders on top, dark borders on the botton. The dark edges seem to follow these unwanted "shadows".
(Sorry for the low quality. I only have an Amateur license and am limited to 2048 size textures)
It also seems like this testing brush has the same problem even with bigger radius. I'm practically convinced this is due to some sort of setting I'm overlooking in the software.
brushtest.psd