Final mission didn't save?

Opened Frontiers for the first time since last year, and found that I could not start new game plus because it had not saved after I completed the final mission. My save file is somewhere near the end but not post-ending. I find this frustrating cuz I don't really want to have to finish the last mission in order to start the game again. Has this happened to anybody else? Any advice welcome, so I'm guessing I'm just going to have to replay the end of this final mission :/