Tangents are not transformed when combining meshes

(Picture is unrelated, but required)

When combining meshes of each part into one, vertex positions and normals are transformed from one object space to the other, but the tangents are copied as is.

This is in the PartGroupScript's CombineMeshes method, and a possible fix is the following (note that handedness is preserved):

Vector3 transformedTangent = matrix4x.MultiplyVector(PartGroupScript._combineSource.Tangents[i]).normalized;
PartGroupScript._combineTarget.Tangents[num2] = new Vector4(
    transformedTangent.x,
    transformedTangent.y,
    transformedTangent.z,
    PartGroupScript._combineSource.Tangents[i].w
);