When working in the Develop module on adjustment brushes, and ONLY with the mask overlay on, the secondary display sometimes does not show the effect. (Toggling the overlay can also cause the brush's effects to not be visible on the secondary display as well.)
I've been working with a lot of images that need corrected exposure. I've adjusted most of my images down about -0.38. When I paint that as an initial brushstroke, the secondary display shows my changes, but adding additional brush strokes, (again, only with the overlay mask on in the develop module on the primary display,) causes the secondary display to brighten or darken on the areas affected by the brush.
I've since noticed that any effect, even with a static exposure will cause the phenomenon of the brush either brightening on the secondary display, or darkening.
You can add a stroke: exposure goes bright. Add another stroke (doesn't matter where, the bright/dark effect occurs brush-wide), and the image goes dark to reflect the proper setting. Add another stroke again, back to bright.
Only when the overlay mask is on in the Develop module in the primary display
With or without Open GL enabled. I've tried both.
In Windows 7x64 and Windows 8x64, I've tried both. (Just upgraded actually, problem was occurring in Win 7 too.
On my XFX Radeon 5870, 16 GB RAM, Intel Core i7 2600K system
Before or after optimizing the catalog, not that that would matter
With any effect at all. Exposure, saturation, clarity, combinations, singularly, anything at all.