Try the solution found in the this thread:
I suspect it is the same issue.
Edit: I understand that this doesn't solve the "lightweight greyed out" issue, it might solve the display issue.
Weird. It works. Thanks!
I know I was doing this part of the time since my assemblies would come up correctly sometimes, but incorrectly other times. I thought it had to do with how I was opening the part (straight from directory vs ctrl + O and double clicking from the directory vs ctrl + O vs typing the file name into the "open file" bar), but this wasn't producing consistent results. Turns out it's the isometric -> forced rebuild -> save that I wasn't doing on a consistent basis.