Greetings, all!
I discovered that if I quit the game using alt-F4 while the "Saving" pinwheel is going, the game is NOT saved! If using alt-F4 to save, pweath have the program wait until the save completes (meaning completely fails or succeeds) before closing!
I'm aware this could be useful in Honour Mode, but that's such a niche case I'd prefer not to strongly consider it.