This seems to happen when the mouse is moved past the end of the visible scroll bar.

== Code
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.Slider;

var lval = 1.0 on replace {
    println("new value = {lval}");
}

Stage {
    title: "Slider Knob bug"
    width: 250
    height: 80
    scene: Scene {
        content: Slider {
            max: 1.0
            value: bind lval with inverse;
        }
    }
}

== Output
....
Adjusting value to position 0.99248123
posValue = 0.99248123
new value = 1.0
Adjusting value to position 1.037594
posValue = 1.037594
new value = 0.0