I am running on Windows, same as you. And again, to emphasize, the problem is NOT in the loading of saves itself. The problem is specifically located in the UI. I'll describe the situation as best I can:
When I'm playing the game, and I want to save, I press the right mouse bottom to summon the save menu, and I save. Everything's fine so far. Now, if I want to load a different save, I click on the 'LOAD' button/icon/whatever, on the left side of the screen. The one below the 'SAVE' button, also on the left side.
Now this is where the shit hits the fan. When I click on the 'LOAD' button, what's supposed to happen is that the word 'LOAD' becomes highlighted AND, the word in the top left corner (in pink) is supposed to switch from 'SAVE' to 'LOAD'. All of this to indicated that you've switched from SAVE to LOAD mode. However, while it does do the former, it doesn't do the latter. Meaning that (in your half jerking-off haze), you think that you've switched from SAVE to LOAD mode, when you haven't. Which means that when you try/want to load that other save, you instead overwrite it with your current playthrough. Which is what has happened to me at least a dozen times over the last year or so.
So that's what happens. I remember reading comments from other players on this forum, when I last posted about this problem, who said they'd experienced the same thing. Off the top of my head, I think that was about a year ago. And the problem still hasn't been fixed.
Based on the fact that this has never happened with any other game, ever, (and I've played well over a thousand) I can only come to the conclusion that this is a bug, originating from 3Diddly. And therefore only he can fix it. I really hope he does.