Why are some replies between commenters left alone?

When a comment starts by tagging another commenter, or sits under someone else's comment in a thread your Page hasn't joined, it's people talking to each other. Herrald still reads it, so a crisis, abuse or a legal matter still reaches you, but it doesn't bring the thread back to your queue, and long runs of it fold away in the thread.

You can turn this off for a brand on the Policies page, and you can always ask for a draft if you want to step in. When a thread turns into a back-and-forth, Mute thread leaves the whole thread to them.