Hello, I experienced the same after managing this issue: "Re-enable member after bouce processed disable" https://lists.mailman3.org/archives/list/mailman-users@mailman3.org/thread/X... after upgrading frm mailman to mailman3 going from bionic to focal.
I also thought that "Connection lost during _handle_client()" was a problem. Anyway sporadically mails sent to a list seem just to disappear. They just appear in smtp.log. Postfix logs show incoming, but no outgoing mails resulting from it. Maybe lost information on tcp socket?
Think I have to enable debug logs to see whats going on. In the meanwhile I instruct mailing list members to ackknowledge own posts to avoid communicaiton failure. Because there are possible problems with hyperkitty setup I have to cross check this issues too.