do real activitypub implementations actually implement inbox forwarding correctly?

if yes, fixing the ghost replies problem with followers-only threads (where if Bob replies to Alice’s followers-only post, and Alice then replies to Bob, people who follow only Alice and not Bob would see Alice’s reply to Bob but not Bob’s reply to Alice) would seem to be as simple as adding the original poster’s followers collection to any replies: why has nobody done this?

if no, why not? (this is something that should be fixed I think)