I just experienced this issue. My steps were:
playing on iphone
1. was doing some event management
2. Attended a friend’s party
3. More event management and started having a baby with one of my 4 sims
4. went and did irl things for a couple hours
5. came back
result:
Immediately i noticed that my sims had lost progress. The sim that was having a baby was now back to being marked as having finished an event instead.
I started checking events then I, foolishly perhaps, made the mistake of checking the party and tried to send a sim back into it.
I hung on a loading screen when i gave up and killed the app and came back in the button to join party was grayed out. It says i already did all my parties for today.
no more time travel, ok server guys?