This happens if you move out a sim from your family to a new lot by themselves. It seems to reset the collections. I have found a workaround for it.
When you have a sim ready to move out of the home, merge them to another household instead of to a new lot by themselves. Make sure it is a family that you aren't playing and don't care if their collections are messed up. From there you can then move your sim to a lot by themselves.
I had my entire legacy collection disappear once and it was terrible. As long as you follow this procedure, you shouldn't have it happen to you again.
