Pressing mute on touchbar doesn't affect volume slider

Describe the bug
I have a "mute" touch bar button and a volume slider widget on the touch bar. If I change my volume via the menu bar, then the slider updates in realtime. However, when I press the mute button on the touch bar, my volume does actually mute, but the slider does not update.

Affected input device (e.g. MacBook Trackpad, Magic Mouse/Trackpad, Touch Bar, etc.):
Macbook Pro Touch Bar

Device information:

  • Type of Mac: Macbook Pro 15"
  • macOS version: 10.13.6
  • BetterTouchTool version: 2.643