Hmmm, I use Windows 7. In the course of the day, I open many, many files hosted by server 13 using that keyboard shortcut.
I have yet to see this. Maybe I'm just lucky or maybe there's some key difference here...
The graph will be created by the client - what client version, what OS?
If You close the graph-window and reopen it: Still no data?
I also just had something very similar happen yesterday, with FMP13.0v5 on OS X 10.8.6, with in a database I open over the WAN, and not the best connection (not the way I like to work, but that's the access I have right now in this case).
Closing and re-opening the Manage Databases window brought the TOs back, but this is certainly a potentially lethal bug. There's areal risk for heart attacks.
If anybody can reproduce this in a test file, it would be interesting to see what happens if you add a TO with the same name as an existing one. I'm hoping this is just a display issue of some sort, so that such duplicates would not be possible to create.