A "discard if unchanged" preprocessing item should be re-recorded to the database when it is removed because it has reached its history/trends lifetime(s) . (it should also be removed from the heartbeat/value cache, resulting in it getting re-recorded the next time it is polled). (removing the item from the database should be considered a change).

workaround: use discard with heartbeat, or restart zabbix, or use really long history/trends times for these items.