This works as intended: the followers are added in Cc only for top level items.
Replies don't get sent to Followers.
Actually only now it's fixed.
The bug I suspect was triggered by the corner case of replying to a comment and having a mention in the comment to the parent's author.
This was triggering an error in the add to collection mechanism, which was erring and stopping execution when the IRI of the reply was already present in the parent's author Inbox. See these fedbox and go-ap/processing commits for the fix.