Hey, according to the information you provided, it seems like you are setting the vertex buffer positions each time you rotate the main model. If not, you can change the vertex buffer type of instancing meshInstance from pc.BUFFER_STATIC to pc.BUFFER_DYNAMIC. And then get the vertex position each time and simply set the latest positions to the vertex buffer.
If you are already doing it and vertex positions are not correct then try using a different method to get vertex positions provided in this thread Vertex World Positions