@chartier well, it's due the decentralized nature of ActivityPub. What happens when you create or edit a post, is that your instance contacts the instance of everyone who follows you and sends it to them.
If you post from instance A and my friend sees the post on instance B and then boosts it into my instance C, I see it. Then you edit it, and your instance contacts the instance of everyone you follow to send the update. But since no one on my instance follows you, your instance won't know to send an update, leaving the original to stay as-is.