ADDITIONAL SYSTEM INFORMATION :
All java versions

A DESCRIPTION OF THE PROBLEM :
In javafx.scene.shape.TriangleMesh.java the calculation of centroid is wrong:
    private Point3D computeCentroid(
            double v0x, double v0y, double v0z,
            double v1x, double v1y, double v1z,
            double v2x, double v2y, double v2z) {

        return new Point3D(
            v0x + (v2x + (v1x - v2x) / 2.0 - v0x) / 3.0,
            v0y + (v2y + (v1y - v2y) / 2.0 - v0y) / 3.0,
            v0z + (v2z + (v1z - v2z) / 2.0 - v0z) / 3.0);
    }

The equation in this code is missing a factor of 2.
    private static Point3D computeCentroid(
            double v0x, double v0y, double v0z,
            double v1x, double v1y, double v1z,
            double v2x, double v2y, double v2z) {

        return new Point3D(
                v0x + (v2x + (v1x - v2x) / 2.0 - v0x) * 2 / 3.0,
                v0y + (v2y + (v1y - v2y) / 2.0 - v0y) * 2 / 3.0,
                v0z + (v2z + (v1z - v2z) / 2.0 - v0z) * 2 / 3.0);
    }
The routine should be:
    private static Point3D computeCentroid(
            double v0x, double v0y, double v0z,
            double v1x, double v1y, double v1z,
            double v2x, double v2y, double v2z) {

        return new Point3D(
                v0x + (v2x + (v1x - v2x) / 2.0 - v0x) * 2 / 3.0,
                v0y + (v2y + (v1y - v2y) / 2.0 - v0y) * 2 / 3.0,
                v0z + (v2z + (v1z - v2z) / 2.0 - v0z) *2 / 3.0);
    }

Alternatively, one can just do:
        return new Point3D(
                (v0x + v1x + v2x) / 3.0,
                (v0y + v1y + v2y) / 3.0,
                (v0z + v1z + v2z) / 3.0);

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Calculate centroid of simple triangle with vertices (3, 0, 0) (-3, 0, 0) and (0, 6, 0)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Centroid is (0, 2, 0)
ACTUAL -
Centroid is (1.5, 1.0, 0)

---------- BEGIN SOURCE ----------
package test;

import javafx.geometry.Point3D;


public class WrongCentroidResult {
    
    public WrongCentroidResult(){
    }

// WRONG
// code from javafx.scene.shape.TriangleMesh
    private Point3D computeCentroid(
            double v0x, double v0y, double v0z,
            double v1x, double v1y, double v1z,
            double v2x, double v2y, double v2z) {

// Point3D center = v1.midpoint(v2);
// Point3D vec = center.subtract(v0);
// return v0.add(new Point3D(vec.getX() / 3.0, vec.getY() / 3.0, vec.getZ() / 3.0));

        return new Point3D(
            v0x + (v2x + (v1x - v2x) / 2.0 - v0x) / 3.0,
            v0y + (v2y + (v1y - v2y) / 2.0 - v0y) / 3.0,
            v0z + (v2z + (v1z - v2z) / 2.0 - v0z) / 3.0);
    }


    private Point3D computeCentroid_Corrected(
            double v0x, double v0y, double v0z,
            double v1x, double v1y, double v1z,
            double v2x, double v2y, double v2z) {

        return new Point3D(
            v0x + (v2x + (v1x - v2x) / 2.0 - v0x) * 2 / 3.0,
            v0y + (v2y + (v1y - v2y) / 2.0 - v0y) * 2 / 3.0,
            v0z + (v2z + (v1z - v2z) / 2.0 - v0z) * 2 / 3.0);
    }

    private Point3D computeCentroid_Alternative(
            double v0x, double v0y, double v0z,
            double v1x, double v1y, double v1z,
            double v2x, double v2y, double v2z) {

        return new Point3D(
            (v0x + v1x + v2x) / 3.0,
            (v0y + v1y + v2y) / 3.0,
            (v0z + v1z + v2z) / 3.0);
    }
    
    public static void main(String[] args){
        System.out.println("expected = Point3D [x = 0.0, y = 2.0, z = 0.0]");
        WrongCentroidResult wcr = new WrongCentroidResult();
        Point3D wrong = wcr.computeCentroid(3, 0, 0, -3, 0, 0, 0, 6, 0);
        System.out.println("wrong = " + wrong);
        Point3D corrected = wcr.computeCentroid_Corrected(3, 0, 0, -3, 0, 0, 0, 6, 0);
        System.out.println("corrected = " + corrected);
        Point3D alternative = wcr.computeCentroid_Alternative(3, 0, 0, -3, 0, 0, 0, 6, 0);
        System.out.println("alternative = " + alternative);
    }
    
}

---------- END SOURCE ----------

FREQUENCY : always