NFSv4 hangs on 13.3

In an attempt to narrow down the problems we have with NFS v4.2, we've
set up a FreeBSD 13.3 server that mirrors some read-only data from our
Linux NFS servers.

We are still observing occasional hangs on the FreeBSD 13.3 client machines.

When these hangs occur, anything that touches the mountpoint
(including "umount -N") hangs indefinitely. There is nothing in dmesg.

The only even slightly informative data I could gather is an "ls
/mount/point" kstack:

$ sudo procstat -kk 94676
  PID    TID COMM                TDNAME              KSTACK
94676 976876 ls                  -                   mi_switch+0xbf
sleeplk+0xea lockmgr_slock_hard+0x3a5 nfs_lock+0x29 vop_sigdefer+0x2a
_vn_lock+0x47 vfs_cache_root+0x9d vfs_root_sigdefer+0x35 lookup+0x88a
namei+0x24a kern_statat+0xf8 sys_fstatat+0x27 amd64_syscall+0x110
fast_syscall_common+0xf8

During this time, the NFS server continues serving other clients, and
the affected client can access other NFS servers without hanging.

The "nfsstat -m" for this mountpoint looks like this:

nfsv4,minorversion=2,oneopenown,tcp,resvport,nconnect=1,hard,cto,nolockd,sec=sys,acdirmin=3,acdirmax=60,acregmin=5,acregmax=60,nametimeo=60,negnametimeo=60,rsize=65536,wsize=65536,readdirsize=65536,readahead=1,wcommitsize=16777216,timeout=120,retrans=2147483647

with these mount flags in /etc/fstab:

ro,nfsv4,minorversion=2,tcp,nosuid,noatime,nolockd,oneopenown

Is there anything more we can do to help identify this issue?

Thanks for any advice!