Not sure when this started.
After upgrading and rebooting, showmount client list empty even though pre-existing mounts are all working perfectly. List repopulates as clients disconnect/reconnect.
FreeBSD get.unhooked.net 12.0-CURRENT FreeBSD 12.0-CURRENT #10 r333990: Mon May 21 21:28:25 PDT 2018 email@example.com:/usr/obj/usr/src/amd64.amd64/sys/TOM amd64
Did it work before? Do you have some idea of when?
That's just how NFS <= 3 works. The client list is only updated by mountd(8), and clients only talk to mountd(8) when they mount or unmount a filesystem. If the server rebooted but the filesystem handles haven't changed, they might not even notice that anything has happened.