We have two servers, active-pasive, sharing a file system, which is only mounted in the active node.
When server A have that FS mounted, it is reported to zabbix, via LLD, and data recolected.
If that FS is umounted, the item "vfs.fs.size" returns the data from FS where that path exists ("/" in our case).
In Linux we see that info is recovered from statvfs, which produces that behaviour.
vfs.fs.size does not return any value if the path is not a mount point
This behaviour is observed with all Zabbix versions