When copying a model, model 1 and model 2 have duplicate models with the same geometric shape. When adding thin instances to the model, when modifying a model 1, the instance content of model 2 will be modified at the same time.
It is expected that thin instances are shared when you clone a mesh. If you don’t want this behavior, you should call makeGeometryUnique() on the cloned mesh.
modelClone | Babylon.js Playground (babylonjs.com)
This is my model clone, which includes the processing of the instancedMesh. It might work for you.
You can use this example directly if you are only modifying the geometry used by the mesh and not the geometry itself. If you need to modify the vertices of the geometry, you also need to clone the new geometry for the cloned mesh.