Question Discrepancy in Skinned Mesh Deformation between Blender and Unity

During a range of motion stress test for one of my models I realized that under certain conditions it deforms differently than what blender is showing me. The blend weights for the affected vertices seem to be identical and I also checked that the use of 4 skin weights is enabled in the quality settings as well as in the model's asset file itself. The animation type (legacy or generic) also does not seem to make a difference. I am using Unity 2021.3.6f1.

Here is an illustration attempt:


The white quad in the middle and the red quad both are much more stretched in Unity than in Blender. I added non-skinned meshes indicated by the blue circles to better show the difference.

I also attached a ZIP file that contains the Blender file used for the screenshot above. It also contains an FBX file if you prefer that.

Can someone verify that this happens on other systems as well? Or is there something I am doing wrong?

Here are the asset file settings I used:



Tested Unity versions so far (where the issue also appears):

2021.3.6f1
2020.3.26f1
2018.3.14f1

 

I now used a different program to render the FBX model file to see if the problem lies somewhere else. Unfortunately, the online 3D viewer at https://www.creators3d.com/online-viewer actually shows the same issue.

That made me try to export my model as a *.dae file and that actually made the problem disappear (although it in turned created some other glitches). So I guess the cause for the error is somewhere in the FBX exporter, isn't it? Does this make it a blender bug then?