The same happened to me for a while. In my case is the sim's mother.
It used to be normal till once I played as another sim leaving the house during her stay-over event.
I switch back home way after the stay-over event period, but her luggage stays in my house.
I guess the game thinks she is still staying over ever since.
Although I can invite her by another family member. This bothers me a lot.
To fix this problem, the way I found is pretty terrible.
You have to delete the relationship between the sims, then add the relationship back.
It is very immersion-breaking and involves cheats or mods, but this is the only way I could fix the problem.
