There seems to be an issue with brightness values on the sliders. Opacity seems to be the offender here, most likely caused by the sprite being drawn twice on a slider start circle after testing.
This test skin, which has hit circles set at 50% opacity, a slightly larger sliderstartcircle set at 50% opacity, sliderendcircle set at 50% opacity, and skin-ini values to give maximum contrast, should highlight the problem.
Test Skin
These two images nicely show the effect.
After failing,
This test skin, which has hit circles set at 50% opacity, a slightly larger sliderstartcircle set at 50% opacity, sliderendcircle set at 50% opacity, and skin-ini values to give maximum contrast, should highlight the problem.
Test Skin
These two images nicely show the effect.
After failing,