We just started using zabbix proxy for about 3 months ago.
When we have deployed (Monitored by proxy tab on host), and restart the zabbix agent on the given server, every zabbix agent item like 'agent.ping' gives no data. Userparameters, still seem to work.
If we then redeploy(Save no proxy, and then puts the same proxy on again), it seems to work again.
We have seen this happen on 3.2.x and now on 3.4.x
We now run with 3.4 on the proxy, server and on the client agent.