Sims 4 my sims are disappearing!

I've had three Sims disappear now, too. They are NOT dying. Their household panel is still in the Household Managment control panel, and it says they are still in their worlds. If I had known they disappeared, I wouldn't have saved (I save often, to a particular game our forum is working on). This has happened with three different Sims, by three different people, in three different houses.

If I try to move that Sim back in (by use of the Library), it ask me if I want to MERGE the housholds, but there is not a Sim there with whom to merge the same Sim (sigh). In the Household Management control panel, the Sim's panel tells me I can't move that Sim out, since it doesn't exist...yet the panel still exists.

The only thing I can do is delete the Sim through Household Management, then move in a fresh copy of the Sim from the Library.

My Sim this time it happened is a write, who had already written several books. I had put those books into the bookcase, since she seemed to want to leave them everywhere she went, rather than keeping them to self-publish or publish (which I just figured out, finally). So when I deleted the non-existing Sim and moved in the same sim via the Library, the house still held her previously-written books.

I thought I'd try it, and sure enough, she was then able to publish those books, even though she, herself, had never written them. They had her name on them, in the bookcase, and when they were put into her inventory and she ran out to the mailbox, bingo! She was able to self-publish the books! That was nice, since she wasn't able to reclaim anything else from her previous life! (another sigh)

A friend of mine has had the same thing happen...Sims just randomly disappearing out of her world. I believe it's definitely some sort of a glitch.