Missing replies in thread view after replying to non-latest messages

Description: When an agent replies to a message that is not the most recent in a thread, the reply is delivered and visible in the agent's message history, but does not appear in the thread panel UI on the right side.

Steps:
1. Start thread with message A
2. Agent replies to A (appears in thread)
3. User adds message B to same thread
4. Agent replies to B
5. Agent's reply to B is missing from thread panel UI (though it exists in relay)

Expected: All replies appear in thread panel, chronologically ordered
Actual: Replies to earlier messages become invisible in UI

Environment: Buzz Desktop