Description:
The brightness slider widget on Touch bar appears to go up to about 90%. This actually represents maximum screen brightness. When you drag the slider to max, it stays there for a second, before jumping back to 90%. The real brightness of the screen doesn't change during that action.
I've tried deleting it and putting in a default brightness slider widget, same issue. This doesn't happen with the volume slider widget.
Affected input device: Touch Bar
Screenshot:
Device information:
- Type of Mac: MacBook Pro 15-inch, 2018 (Radeon Pro 560X)
- macOS version: 10.14.1
- BetterTouchTool version: 2.660
GIF of bug:

The BTTPreset file:
KISS Bar Modified.bttpreset (404.2 KB)
This is due to the chosen icon, the slider widget does not account for a differently sized icon than the one provided. I have the same issue with my sliders since I'm using a smaller icon than the base one.
It even happens with no icon just using the default rounded rectangle.
after requesting a brightness from the system, BTT checks what brightness has actual been set and adjusts the slider value to that.
Maybe some screens don't accept the 100% value? Is this adjusting the brightness on your Macbook?
Did anyone ever get this sorted out? I have the exact same issue on a MBP 16" with Catalina.
Just installed BTT for the specific reason of having the brightness- and volume sliders open at all times, but the brightness slider won't stay all the way to the right even though brightness of the screen is 100%.
In my case it is the brightness slider widget that comes with BTT that exhibits this behaviour, on the laptops own screen.