I loaded 56-2 (or 56-1, not sure anymore) and left that menu, then saved in the overview, overwriting 56-2.
Aka, the later saves load a save that doesn't even exist anymore. Or maybe they just saved the same thing, and the difference in file size is just the preview picture. It did occur to me that that might make troubleshooting harder, but sadly only after overwriting the save. The original would have been around 19:30 - 19:35 my time.
Thanks for reminding me of the secondary save location. Curiously, the new 56-2 doesn't exist in the game files at all.
Lacking the original, I've added one of the later files that now leads to that earlier save, and an autosave that even says it does, even though it's been generated later. Also, the save from before the issue (56-1).
I just tried again, and while saving there, and then later somewhere else, shows the living room background while saying "loaded", it then jumps to the actual save location and time.
I'll try to reproduce this properly tomorrow.