All I can tell is that's not an usual issue. Never heard of problem of this kind before… So yeah, there is something weird. Could be related to NFS permissions maybe, or something between XCP-ng and your NFS host. It's not XCP-ng code directly, because otherwise everyone else using NFS will endure the same problems. So it's more a "combo" between XCP-ng and your NFS within your infrastructure.
Please read the wiki on where to check logs (especially SMlog) to spot potential hints on where's the issue.