Steps to reproduce:
1. Make sure you have your Proxy in active mode
2. Check the HeartbeatFrequency= parameter.
3. Add host to be monitored by proxy
4. Add an item with the Update interval less than HeartbeatFrequency value.
5. Observe the results under Monitoring -> Proxy tab.
6. Compare the observations from Monitoring -> Proxy with the Last 500 Values tab for tested item.
Last seen will be updated only when the next Heartbeat message would be sent.
Last seen needs to be updated as fast as new bunch values will came in.
P.S. - Behavior is the same for both - Zabbix Agent Active and Passive item checks.