I have some additional info on this problem that I think is relevant.
If I use the tea set frequently, it doesn't seem to disappear. It only disappears if I go a day or two without using it.
Could just be coincidence, but at this point it's pattern that was worth mentioning.
I have only had the set disappear in a game with mods, so I can‘t share my file. What I noticed in a game without any mods though is, that sims frequently prepare tea without pouring it, the tea then spoils and the tea set becomes a dirty object that gives sims the „dirty surrounding“ moodlet or make them do the corresponding animation. Because the set doesn‘t have a smelly cloud over it when dirty, this can go unnoticed for a while which matches @JaggidEdje s observation. At the moment I’d guess maids or the game (on household swap) clean what they consider to be spoiled food or trash.
Also as mentioned by someone above, it sometimes doesn‘t seem to clean properly and makes sim react to it as if it was a dirty object even though it was recently cleaned. I tested this by dragging it in and out of the room in which my neat sim was currently reading.
