The Linux file system discovery code uses '/proc/mounts' contents as the source of the mounted file systems list.
While this represents kernel's true view of the available filesystems, it quite often differs from the view of user space utilities (like 'mount' or 'df'), which read '/etc/mtab' instead.
One of the problems is listing automatically mounted NFS snapshot volumes (these never make it to '/etc/mtab') - and triggering 'file system full'-sort of checks that many Zabbix users define.
Thus, it might be preferrable to choose '/etc/mtab' as the source for the discovery.

An agentd configuration option letting users choose the discovery source would solve such problems. One could specify:

FSListSource: /etc/mtab

With the default being still like:

FSListSource: /proc/mounts

You will find a proposed patch for 2.2.2 attached.