I remember when Twitter worked like this as well. It doesn't any more. When I see a tweet with a dozen replies and only one is visible, even in a private tab, it's very unlikely that eleven out of twelve are from locked accounts. It's even less likely that there wouldn't be at least one visible reply to one of those replies. I see this sort of thing often enough that I have no doubt that twitter is futzing with the visibility both of certain posters and of interactions with certain posters.