One minor possibility is that those posts' only content are mentioned usernames and there's nothing else. The algorithm may have flagged them as suspicious, which makes more sense.
Highly unlikely. What the algorithm might do is probably, not showing the post to people and make you feel invisible (i usually do) but it won't be selective like this, hiding some posts which are part of a thread. It's probably a glitch or something.
No I don't. What I mean is; if that account isn't public, then it means those posts are protected and only the followers of the account can see them. If that's the case, there was nothing to report.
That usually happens when the author's account (that replied) is not public/protected and only visible to followers or the ones that the account follows.