In FreeNAS. exports file you should have fsid=123, with a unique number for each exported path. If not it can cause a problem if the nfs server is rebooted while the pool is up.
https://linux.die.net/man/5/exports
https://www.suse.com/support/kb/doc/?id=000017897
The stale mounts are in xcp-ng.
But this is just a theory.
During the power outage. Did all xcp-ng hosts and the FreeNAS as boxes go down? How did you boot up? You wrote you set up fresh servers?