Hi!
It seems that there is some issue with how meshes with applied multimaterials react on freezing active meshes on the scene. Check out the playground - I used the example of multimaterial from the docs, and I do scene.freezeActiveMeshes(); after 3 sec. timeout.
From what I’ve seen so far, in the render method of Mesh class the following code sets _effectiveMaterial specific to the sub mesh:
// Material
if (!instanceDataStorage.isFrozen || !this._effectiveMaterial) {
var material = subMesh.getMaterial();
// ...
this._effectiveMaterial = material;
// ...
}
Presumably this needs to be done for every sub mesh of a mesh, but the problem is that _effectiveMaterial is a shared property, therefore as soon as active meshes get frozen, the instanceDataStorage.isFrozen becomes true, and _effectiveMaterial “stucks” with one material, so that other sub meshes of a mesh never render with their materials.
Not sure if I make sense though 
Tested BabylonJS version: 4.0.0-rc.3
