I have no clue what causes this proble but reseting all brushes and playing with brush settings doesnt help, but brush behavior feels like it works only EITHER in x/y/z not all at once as it naturally does
what’s weird is that the issue only appears in 64bit version of zbrush, regular 32bit runs flawlessly
First image is brush snap angle, second is an effect.
I could be totally wrong… but if I had to give you answer based only on what you said… then I would say, check to see if you are using symmetry and if its on the wrong axis.
To me, if I observe the image you provided, it looks like you are applying a smooth brush or subtract clay build up with very low intensity. I see nothing to suggest in the image that the axis is off.
How could I explain it… it’s not of those bugs that are easy to show once you understand what’s going on, pretty much when i take brush and put it at any angle, it only works in single axis.
It’s especially noticeable with trim and polish brushes since they simply dont work… i guess i will try to record th video 
update
its messed up, i’ve opened x64 just now and problem seems to be gone o.O
Although the images above describe:
When I take brush and put it next to the edge it did not calculate the angle under which the brush was supposed to be used at, but instead it snapped to the closest axis