Material vertexTangents should be a boolean.

examples/js/loaders/GLTFLoader.js

@@ -2829,7 +2829,7 @@ THREE.GLTFLoader = ( function () {
 									: material.clone();
 
 								if ( useSkinning ) cachedMaterial.skinning = true;
-								if ( useVertexTangents ) cachedMaterial.vertexTangents = THREE.VertexTangents;
+								if ( useVertexTangents ) cachedMaterial.vertexTangents = true;
 								if ( useVertexColors ) cachedMaterial.vertexColors = THREE.VertexColors;
 								if ( useFlatShading ) cachedMaterial.flatShading = true;
 								if ( useMorphTargets ) cachedMaterial.morphTargets = true;

src/constants.js

@@ -14,8 +14,6 @@ export var BackSide = 1;
 export var DoubleSide = 2;
 export var FlatShading = 1;
 export var SmoothShading = 2;
-export var AutoTangents = 0;
-export var VertexTangents = 1;
 export var NoColors = 0;
 export var FaceColors = 1;
 export var VertexColors = 2;

src/materials/Material.js

 import { EventDispatcher } from '../core/EventDispatcher.js';
-import { NoColors, FrontSide, FlatShading, NormalBlending, LessEqualDepth, AddEquation, OneMinusSrcAlphaFactor, SrcAlphaFactor, AutoTangents } from '../constants.js';
+import { NoColors, FrontSide, FlatShading, NormalBlending, LessEqualDepth, AddEquation, OneMinusSrcAlphaFactor, SrcAlphaFactor } from '../constants.js';
 import { _Math } from '../math/Math.js';
 
 /**
@@ -24,7 +24,7 @@ function Material() {
 	this.blending = NormalBlending;
 	this.side = FrontSide;
 	this.flatShading = false;
-	this.vertexTangents = AutoTangents; // THREE.AutoTangents, THREE.VertexTangents
+	this.vertexTangents = false;
 	this.vertexColors = NoColors; // THREE.NoColors, THREE.VertexColors, THREE.FaceColors
 
 	this.opacity = 1;