I don't have an answer for you but some data that might help your debugging:
- I'm still learning, but I'm not aware of anything you can do as a regular user to hide/lose posts on your profile.
- I've been actively tracking new user questions in popular Lemmy support communities and I can say this is not a common report over the last week. You're not stumbling on some frequent trap that new users hit. It's weird.
- If no one else chimes in with advice, I'd consider talking to you admin. Is there a meta community local to your server? Or your server's admin(s) should be listed on the sidebar of its homepage. You could at-mention them to ask if they have seen any errors from Lemmy or Postgres.