I think this is a Webclient (Flex / Flash) bug.
We are having this problem with vSphere 6.5 and NFS datastores from time to time, too.
This looks like a nasty bug in the vCenter Web Client. The old Windows based vSphere client ran a rescan on each host after changing a datastore size, which the Web Client doesn't seem to do. Because of this, the vCenter Server gets different size information from the hosts, and seems to display the size of a random host.
What you can do is to run a rescan on each host while either logged in directly on the hosts (not from vCenter), or by using a script in case of a large environment. This will make each host aware of the new size, and vCenter Server will get consistent size information.
I'll give your method a shot, though if I rescan it seems to be okay for a while and then it will periodically detect/alert the previous size. I'll find out when we'll migrate to the 6.7.0 client and maybe that'll resolve the issue. Thank you.