Normals are incorrect when applying a blendshape to a skinned mesh

[Arnklit]

Godot version:
3.2.3.Stable

OS/device including version:
Windows 10 Pro v.2004 / Inspiron 7373 Core i7-8550U / Intel(R) UHD Graphics 620

Issue description:
Normals are incorrect when applying a blendshape to a skinned mesh. This is a glTF imported model. I tried to compare to an fbx file to see if it was an import issue, but the blendshapes don't seem to affect the normals at all on fbx imported models.

Steps to reproduce:
Open attached project and observe the normals when applying the blendshape

Minimal reproduction project:
glTF_blendshape_normal.zip

EDIT: I added details about this being a glTF model in description

[Calinou]

Related to #42620.

[RevoluPowered]

Question, does this happen with stuff which is not FBX too?

[Arnklit]

This happens with glTF. It does not happen with fbx, but fbx has the error of not adjusting the normals at all when a blendshape is applied. I have not tried with the new fbx importer that I think is in beta now in 2.3.4 and I have not tried with Collada since the Better Collade exporter for blender doesn't seem to be kept up to date any more.

[Calinou]

Related to #55812.

I can reproduce this on 3.5.beta 11d40d7 (Linux, NVIDIA GeForce GTX 1080):

GLES3

GLES2

In both rendering backends, enabling Flags > Ensure Correct Normals in the SpatialMaterial does not resolve the issue. It does change the normals' appearance a bit, but it doesn't fix them.