FWIW......
I just tried a new test with the "Vanish" game that had been posted
I hammered the buildings until one disappeared then saved normally.
When I restarted, the multi-hex building that had only shown rubble had been restored. ( WITH THE RUBBLE....which is as it should be )
Anyone interested can try that and report that they see the same thing.
EDIT
This was done with MBT but the test I ran with SPWW2 did the same
This is after the multi hex building had been hammered by arty and disappeared
I then saved the game in a regular save slot then restarted
The building returns on restarting the save