When Zabbix Proxy sends data back to the Zabbix Server via stunnel, if the stunnel connection fails to the server, Zabbix Proxy still thinks that the data has been sent. As such, data is lost during connectivity issues between the stunnel client and the stunnel server.

The ids in the ids table on the Proxy also keeps incrementing even if stunnel is reporting "connection reset by peer", with 0 bytes sent. It can be seen in the stunnel logs that Zabbix keeps sending data through stunnel, in line with the items Interval.

Without stunnel, it seems Proxy detects a connectivity issue, and waits a while before trying again - it doesn't just keep trying to send individual items back. Once the connection is back up, it logs "Connection restored.". You don't get that when going over stunnel.

This could be due to that the Proxy is always able to connect to the TCP port stunnel is forwarding - it's running on the same box. The problem is, if the stunnel part fails, Proxy doesn't detect that the Zabbix server didn't get the data, and carries on as if it has.