Thanks a lot for providing the feedback to me!
If you change the minimum frame rate to 0, the parts don't disappear. This is an as designed behavior which did for Inventor 2014 to improve the large assembies view interactive performance.
When you rotate, zoom or pan the big models if it can't reach to the default minimum frame rate, it will cull some small parts to reach the value.
This only happens to rotate, zoom and pan, so when you finish these commands, you should be able see the complete model.
Access a broad range of knowledge to help get the most out of your products and services.