Thank you for checking back, @EA_Kristi.
Needless to say, I was not enthused about the idea of starting a new game and waiting for a week or two to have the chance to test it, but I finally brought myself to it.
This problem and two other problems I noticed in the interim of Sims not being able to eat grilled cheese sandwiches or put birthday candles on cakes, were possible in the new game without mods.
I also went back to the linked thread and realized that although I do not have any of those mods in the game, it's entirely possible that I have rugs that are on the mango-sims mesh, since she has a generous recoloring policy and allows others to include her meshes. That's the only thing I can think of.
What is the solution?
I've been playing one Sim with a culinary aspiration and career on long life, who is about ready to age up to elder. After attending festivals with no problems long ago, aging up to adult, and having no reason to stoop to making grilled cheese for up to 80 days, who knows how far back I'd have to go for saved games to get ahead of this problem!