Encountered the same problem with the primetime yesterday, watched the whole thing, and oull mod wasn't marked as obtained because, ... something, today watched the whole stream too. twitch marked both drops as obtained still no inbox message from either stream.
Edit: Twitch and warframe accounts were linked during stream but for some reason twitch didn't notify DE of the drops. Re-linking accounts helped to resolve the problem, but warframe website is more spaghetti code than the game so it took some convincing button smashing to unlink accounts.