Stellaris Save Changed itself?

Hi, new to subreddit here, but have been playing Stellaris for a long time. So I was wondering if anyone else has ever encountered an issue where they properly saved and stopped playing the game for like a day and then come back. Then noticed changes in what Empires occupy what systems? This happened to me and I know for a fact that said empire did not sneak into some of the open lands in my Empire and occupy.

What I'm really trying to say is has anyone come back to playing a game and noticed one or 2 countries way bigger than they were before?