As reported in element-hq/element-x-ios#4347

On thread aware timeline instances (i.e. timeline foci of Live { hide_threaded_events: true } or Thread { root_event_id) read receipts sent from unthreaded clients (i.e. missing a thread_id) are not considered when computing the user's latest receipts, resulting in incorrect information being reported.

Even though the remote party has read all the messages (on a flat timeline), the current user will remain stuck on their previously sent one, as they were the latest having a thread_id.