The panel plugin keeps an itnernal state of the current brightness. However, that means that if the brightness has been changed by some other source, such as hardware, brightness keys, etc., the plugin's assumption about the current brightness will be wrong. Instead, the plugin should always query the current brightness before showing the slider or making any changes. Example: My monitor is at 80% brightness. In the evening, I reduce the brightness via hardware buttosn to minimum to spare my eyes. I later want to increase the brightness slightly and execute one mousehweel-up event on the brightness plugin. But since the plugin still thinks we're at 80%, it'll blast my eyes with full brightness instead of modifying the actual value.
We've meanwhile dropped the brightness plugin and merged its functionality into the (remaining) battery plugin (which will likely be soon renamed into power manager plugin). The brightness value handling was fixed with this merge.