It really depends. Best would be to look at a save file.
But I see 2 possible things:

1) The game could have been started on previous patch, and the update messed with calculations

2) You may have memorized a specific amount of prestige points a few turns before closing the game. Upon starting the game, you see your prestige go down, because in that few turns you lost a leading position in a specific field (like sector control), which results in a massive prestige lose.

Is that why in previous games, when I'm on a knife's edge diplomatically and then save and quit, when I reload war is instantly declared?  Finally an explanation after all these years