There is a bug in Blender that occurs when attempting to Undo (CTRL+Z) in Weight Paint mode or Sculpt mode.
- Demo: Undo Bug
Workaround: After adjusting the brush values (Weight, Radius, or Strength), switch to a different paint brush (Add, Subtract, Mix, etc.), and then switch back to your previous paint brush.
- Undo (CTRL+Z) should now work as expected.