This is not a bug, the real To is protected with RFC 9788: Header Protection for Cryptographically Protected Email
It is inside the encrypted part. The real list of the recipients that the server uses is in the RCPT TO SMTP commands which are not part of the message. This is similar to how Bcc works.
It is not clear what you tested in each case. E.g. with “DC (Fastmail) → Gmail” I assume you typed in the email address of the recipient? I don’t understand what “but reply isn’t available in DC” means, you mean you sent a reply from Gmail to Fastmail and it is not displayed? This is because of the “Show classic emails” setting likely, you can set it to “All” for Fastmail profile and then reply from Gmail should be received.
As for getting stuck on “establishing connection”, is any message delivered to Fastmail? This sounds like spam filter problem.