Additional information (e.g. StackTraces, related issues, screenshots, workarounds, etc.):
Note: Before bug reporting, please make sure you have the latest version of BetterTouchTool and that you have already tried to restart your system :-). If you encounter a crash, please attach a crash log from the macOS Console.app from the "User Diagnostic Reports" section.
Unfortunately I currently can't test because my Touch Bar system is already on the Mojave DP 5 which breaks button background colors for everything
However it seems to update the values in the widget correctly for me. Are you sure your UUID is correct (and make sure it is a widget, not a normal Touch Bar button)?
To update a normal button you'd need to use the "update_trigger" function.
Take a look at the Microphone Mute button in my preset. A very small modification to it should do exactly what you want. Well, as long as you're not on Mojave DP5