I'm also having this exact same issue on (Updated) ESXi-6.5.0-20170304101-standard (VMware, Inc.) + FreeNAS 11.0-U2.
I can read files created on the FreeNAS NFS share, but I can't delete, create or modify from the WebUI. Haven't tried to run a VM from that storage yet, but should be impossible as I can't write to the share.
Any help would be appreciated.