In the Edit Shared Folder Permission dialog box, when I select a shared folder that I know has NFS host access configured, instead, Users and group permissions is selected.
That’s the UI default. You have to manually select the NFS settings window each time you need it. 
Hi, regarding this issue, please refer to the relevant FAQ for more information and troubleshooting steps: How to access files on NAS via NFS from UNIX/Linux clients? | QNAP
Hope this helps!