Hi,
I had the same behavior when adding nfs datastore via powercli and noticed that I forgot the trailing / on the -path parameter (it correspond to the field "Folder" on GUI).
I had to use /<path-to-nfs-export> and not <path-to-nfs-export>
For me, confusion came from the fact that with netapp cDOT, /vol/ is no more the base export path.