My family doesnt eat in the succah shmini atzeres and when I asked my father why we don’t he said because his father didnt and my great grandfather didnt… but we’re chassidish so it’s our minhag not to. What’s interesting is that the Avnei Nezer brings down that one DOES eat in the succah on shmemini azeres but every year something came up (rain or illness) that prevented him from doing so.