Blendshape changes object color on its own

I use blender to generate a very simple cube, assign it a base color and a blend shape that shrinks the cube in one dimension. After these, I save the cube as a gltf file. Then, in gltf viewer powered by three.js (e.g., https://gltf-viewer.donmccurdy.com), dragging on the blend shape will change the “color” of the cube.

This seems odd. I’m pretty new to this space. Is this a bug or a feature?

I’m attaching the blender python script, the generated gltf file as well as a video clip for the color changing effect.

import bpy

cube = bpy.data.objects['Cube']

mat = cube.data.materials[0]
mat.use_nodes = True
mat.node_tree.nodes['Principled BSDF'].inputs[0].default_value = (0.1, 0.2, 0.3, 1)

sk_basis = cube.shape_key_add(name='Basis')
sk_basis.interpolation = 'KEY_LINEAR'

sk = cube.shape_key_add(name='Key')
sk.interpolation = 'KEY_LINEAR'
for i in range(4):
  sk.data[i * 2 + 1].co.z = -1
for i in range(4):
  sk.data[i * 2].co.z = -1

gltf file: Dropbox - 1.glb - Simplify your life

video clip:

Found the solution: Blendshape changes object color on its own · Issue #283 · donmccurdy/three-gltf-viewer · GitHub