That's strange. I'm sure I've exited out of the screen and gone back and seen my Sim's still at the table eating their meals. Maybe it depends on how long you've been gone because the time does go on even if they don't age. If it's more than a day maybe the game cleans up once the dishes spoil like on commercial lots and it can't tell your dishes won't spoil?
I compulsively put everything in the fridge right away, perishable or not lol. I only leave plates out when there are toddlers so they can grab one on their own if I don't notice their hunger. I do really like that trait though.
I also just discovered if you max the mixology you can freshen drinks so they last longer.
