Fix Silhoutte breaks some meshes in A-Pose

[lyuma]

Godot version

4.0.2-stable

System information

Windows 10

Issue description

I have a model in A-Pose. I was expecting to be able to import this model to Godot using its humanoid retargeter, by adapting it to T-Pose with the Fix Silhouette option. As noted in the documentation, this option "should be enabled for A-pose models.".

However, the resulting mesh looks wrong. In contrast, the resulting skeleton/armature looks correct.

If the mesh deformed with this armature, it should not have been broken in this way, so it must be caused by a bug.

Here is the real world example which inspired this testcase. However, it is a paid asset and cannot be redistributed.
BoneMap but without Fix Silhouette. The bone rotations are not suitable for animation retargeting because the original model was in A-pose.

BoneMap with Fix Silhouette. The animations now line up correctly, but the model's mesh or bind poses now appear to be corrupt.

Steps to reproduce

1. Open the attached project
2. Allow the models to import.
3. In main_scene, compare the models in front (no Fix Silhouette) to the model behind (Fix Silhouette). They should look the same, except one will be in A-Pose and the other in T-Pose.

Minimal reproduction project

SilhouetteFix.zip
Here is the blender model pre-export fix_silhouette.blend.zip

[lyuma]

As an experiment, I rotated the bones so that they are all facing Y-forward. However, the issue still happens, so it should rule this out as the cause.

Here are the blend and glb files with bones facing forward in Y
fix_silhouette_rotated.blend.zip
apose_silhouette_fix2.glb.zip

[TokageItLab]

It seems that for some reason Use Named Skin cannot be enabled and the skin/bone bindings are broken. It may be necessary to fix this so that bone idx is prioritized when Use Named Skin is disabled. Not relevant, looks like I stepped on another bug.

Perhaps the cause is this change #65278, something is not calculated correctly since fix silhouette seems to work correctly in alpha 15.

[lyuma]

Fixed some weight painting issues so each front/back triangle is attached to exactly one bone. It is now easy to tell that RightUpperLeg, RightShoulder, RightUpperArm, RightLowerArm, Neck and Head are really far off. LeftShoulder also looks a little bit offset.

glb plus .import: apose_silhouette_fix2.glb.zip
blend file: fix_silhouette_rotated.blend.zip