This is happening to me, too! It took me a while to figure out but I just realised the issue is caused by adding a second elevator to the lot.
I've been working on a pretty big build but making no progress because out of the blue, the game wouldn't save anymore and I'd have to start over. I've just come to realise the game stopped saving every time I added the elevators, which I confirmed by testing my theory just now. One elevator, the game saves. A second one, the game won't save anymore. No error message, just nothing happens. No "saving" message, like OP said, the menu just disappears and I'm back in the game.
It's definitely the elevator, although I've used this trick in the past without issue, so I wonder what's causing it now.
