Agent upgrade retains discovered filesystem monitoring. Agent Fresh install does not

[andy.torres]

We are running Zabbix server version 4.0.3. For a very long time we have used agent version 4.0.33 on our monitored hosts, things have always worked well.
Some time ago we updated the agents to version 5.0.15-1.el8, and all things continued to run well.
We rebuilt some agent systems and installed the agent version 5.0.15-1.el8 (no upgrade of agent from version 4) and noticed that we see no latest data for mounted filesystems.

How did it retain the same functionality when we simply updated the agent, but does not work when the newer agent is installed fresh?
We frequently rebuild our test environment systems, and we would like to continue to use the version 5 agent. Do we need to upgrade our Zabbix server in order to have everything run correctly?

[cyber]

Generally you should not run agents with higher version than your server is... But then again, with old and tested items it usually causes no harm. You just cannot use items meant for newer version, that this agent probably could use, but your server does not know anything about...
Are those FS items still in "supported" state? Maybe accidentally switched on the "do not keep history" ? Nothing in logs, both agent and server?

[andy.torres]

I created a support ticket with Zabbix. They confirmed that using a Zabbix v5 agent with a Zabbix v4 server will most likely cause this type of issue.
So, we backed down our agent versions to v4 and filesystem graphing showed back up in the list after each Host's discovery rule kicked in.