Hi,
The FreeBSD NFS server is broken when handling multiple
connections for NFSv4.1/4.2.
It incorrectly binds the back channel to a new connection
when is sees an RPC with Sequence in it (almost all RPCs)
and might send a callback on the wrong connection.
I have a fix, but it won't be in a release until 12.3/13.1 (months away)
It can be found in FreeBSD Bugzilla PR#254560.
In the meantime, avoiding use of the "nconnects" option
on Linux client mounts to FreeBSD servers should avoid
the problem.
Thanks go to Ben Coddington @redhat for diagnosing
this and testing the patch.
rick