thanks for the headsup UnrealExpert.
This patch fixes incorrect saving into the slot 0 and the save file issue when you are trying to start a new save at a new difficulty. Previously, the game seemingly tried to save slot 0 regardless of which slot you had picked when you tried to exit to the menu from the level start menu.
So basically, if you started slot 1/2, went into the game, then chose to leave the game without starting the mission, it would save your progress under slot 0, very specific issue. Very dumb one too.