Lemmy is a service for the purpose of publishing your posts & comments to servers that are outside of your control or the control of your local server's operator.
That is literally what it is for. That is not a bug; it is not just a feature; it is the whole design.
The ability of servers to go their own way is why it's a federated service and not a centralized one.
When you send an email from you@aol.com
to them@ox.ac.uk
, you cannot take it back. The content of your email is sent from your computer, to AOL's server, to Oxford's server, to the recipient's email client on their computer. Oxford can back it up as part of regular server backups. The recipient can print it out on their printer and put it in a filing cabinet and show it to someone ten years later. For that matter, AOL is under American jurisdiction and Oxford is under British jurisdiction.
That's literally what SMTP email is for. It's for sending a message to someone else in a different zone of control. As a result, you can't take it back.
Same goes here. ActivityPub is a lot like email, Usenet, and other classic federated protocols; a major difference is that it's implemented on top of HTTPS and JSON rather than raw TCP and line-based classic-IETF-style protocols.