Holes in Mesh only showing from one side - unity3d

Question

For a sailing game I'm working on, I've added functionality to programmatically create damage holes in a mesh (e.g. cannonball holes in a sail). This is largely based on the method here (link to the example code here)
private void MessWithMesh() {
filter = this.transform.parent.gameObject.GetComponent<MeshFilter>();
mesh = filter.mesh;
filter.mesh = GenerateMeshWithHoles();
}
private IEnumerator GenerateTrisWithVertex() {
// Destroying the sail won't work until this has finished, but it only takes a second or two so I don't think anybody will notice.
trisWithVertex = new List<int>[origvertices.Length];
for (int i = 0; i <origvertices.Length; ++i)
{
trisWithVertex[i] = ArrayHelper.IndexOf(origtriangles, i);
yield return null;
}
yield return null;
}
Mesh GenerateMeshWithHoles()
{
float damageRadius = 1f;
Transform parentTransform = this.transform.parent.transform;
Hole[] holes = this.GetComponentsInChildren<Hole>();
foreach (Hole hole in holes) {
Vector3 trackPos = hole.transform.position;
float closest = float.MaxValue;
int closestIndex = -1;
int countDisabled = 0;
damageRadius = hole.diameter;
for (int i = 0; i <origvertices.Length; ++i)
{
Vector3 v = new Vector3(origvertices[i].x * parentTransform.localScale.x, origvertices[i].y * parentTransform.localScale.y, origvertices[i].z * parentTransform.localScale.z) + parentTransform.position;
Vector3 difference = v - trackPos;
if (difference.magnitude < closest)
{
closest = difference.magnitude;
closestIndex = i;
}
if (difference.magnitude < damageRadius)
{
for (int j = 0; j <trisWithVertex[i].Count; ++j)
{
int value = trisWithVertex[i][j];
int remainder = value % 3;
trianglesDisabled[value - remainder] = true;
trianglesDisabled[value - remainder + 1] = true;
trianglesDisabled[value - remainder + 2] = true;
countDisabled++;
}
}
}
// If no triangles were removed, then we'll just remove the one that was closest to the hole.
// This shouldn't really happen, but in case the hole is off by a bit from where it should have hit the mesh, we'll do this to make sure there's at least a hole.
if (countDisabled == 0 && closestIndex > -1) {
Debug.Log("Removing closest vertex: " + closestIndex);
for (int j = 0; j < trisWithVertex[closestIndex].Count; ++j)
{
int value = trisWithVertex[closestIndex][j];
int remainder = value % 3;
trianglesDisabled[value - remainder] = true;
trianglesDisabled[value - remainder + 1] = true;
trianglesDisabled[value - remainder + 2] = true;
}
}
}
triangles = ArrayHelper.RemoveAllSpecifiedIndicesFromArray(origtriangles, trianglesDisabled).ToArray();
mesh.SetTriangles(triangles, 0);
for (int i = 0; i <trianglesDisabled.Length; ++i)
trianglesDisabled[i] = false;
return mesh;
}
When a cannonball hits the sail, I add a Hole object at the location of the impact, and I call MessWithMesh. The holes are often generated correctly, but many times they're only visible from one side of the sail (it looks fully intact from the other side). It's often visible from the opposite side of the sail that the cannonball impacted (the far side, not the near side), if that's at all helpful. The ship I'm using is this free asset.
I'm not really familiar with meshes, so I don't really understand what's going on.