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.

Result:
Last seen will be updated only when the next Heartbeat message would be sent.
Expected:
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.