The "Save" he would need, which no longer exists, is the original "AutoSave_OnEnteringBase" save or the "AutoSAve_OnLeavingBase", if it created one when he attempted to leave. Once he went back to the previous Save and then returned to base, the game overwrote the "AutoSave..." stuff, and he got a different roll of the dice for the next mission assignment.
Parameters understood for both of you guys' experiences with a mission assignment, as well as a fellow earlier who had similar at Fremantle. I am getting my cities mixed-up, kind of like "If this is Wednesday, this must be San Francisco", only to find out later, that it is Thursday, and Chicago instead...