Question Mesh changes during export to assetbundle

We are currently hunting a curious bug where a mesh apparently slightly changes its vertex coordinates (seems like it truncates the floats) when exported to an assetbundle. Unity 2021.3.5f1

The setup:
Two meshes which fit together nicely without any seams. See runtime_editor.jpg

After exporting the scene these are contained into an assetbundle there seem to be apparent gaps between the meshes. See runtime_assetbundle.jpg

Investigation:

• Position of the bottom model doesn't change adding some random cubes as markers shows that seemingly only the top mesh changes position
• Both transforms are exactly at the same position they are in the editor.
• Loading the assetbundle scene inside the editor allows us to swap the top mesh in the meshfilter with the original from the editor. Everything looks fine after setting the editor mesh so the mesh data itself seems to change when being exported to an assetbundle.
• Writing a small mesh compare tool we can see that nearly all vertices have a small amount of vertex displacement in seemingly random directions. See mesh_data_compare.png (A is the assetbundle mesh, B is the editor mesh which the assetbundle is built from). Nearly all vertex coordinates in A seem to be missing some of their least significant bits.
• Putting both of these meshes in a small project for reproduction doesn't seem to exhibit the same behaviour.
• No other mesh in this (rather big) project currently seem to exhibit this behaviour.

Has anyone had a similar experience or any idea why this only happens to exactly one mesh in the whole project.
If anyone has idea on how to track where this issue is coming from I would appreciate any pointers.

 

Maybe it’s due to the setting of that mesh. More information can be found in https://docs.unity3d.com/Manual/mesh-compression.html#mesh-compression