NetBSD's ZFS implementation is not current:
https://wiki.netbsd.org/projects/project/zfs/
ZFS is not my forte, but zpools have particular versions and according to the man page the versions can only be upgraded and never downgraded:
https://www.unix.com/man-page/freebsd/8/zpool/
FreeBSD's ZFS is very current, so it's likely that NetBSD's is too old by comparison and cannot read the newer zpool you have.
For a better answer from someone with a clue - this isn't the place. Try daemonforums.org or ask on the NetBSD mailing lists.