That is correct, in DSC you only get spoils from the secret chests on your initial run, For DSC the regular encounter chests only give spoils on subsequent completions on the same character that week. Yes this is different to Last Wish and Garden of Salvation. For your question of why this is how it is, I can't say, perhaps it's a trade-off as your first DSC completions give pinnacle loot while the other raids do not. But for your question of being owed spoils, you are not owed any as it was working as designed and intended.