Are you backing up the vSphere Inventory Database?
The Inventory service came about as a separate item in vSphere 5.x and is the place where all of your web client tags get stored (you ARE using tags, right?) but it also gets used to cache queries destined for the vCenter database from the web client, which reduces the load on the vCenter server database. It’s also a database that grows substantially if you ever recover a load of VMs with SRM or similar, but that’s another story.