Parts disappear completely?

I've had this happen a few times -- I will create multiple studies, and run just one of them.  After defining the stress plot, I will hide parts in order to see certain areas in the assembly.  If I leave them hidden and go to run another assembly, the parts are missing from the assembly.  They aren't still hidden, they are actually missing.  Then, when I try to close the assembly to (hopefully) preserve some of my results, it asks me if I want to save the changes that I've made to the missing components.  Has anyone experienced anything like this?SolidworksSimulation