FreeBSD NFS client vs. Linux server

[J David]

Hello,

We observed the following odd behavior on a FreeBSD 9.2 client
connected to a Linux NFS server.  Basically, the FreeBSD user's
additional group memberships are not being honored:

$ ls -l

-rw-rw----  1 test2   test2     19188391 Jul 25 03:53 error_log

$ id -a
uid=123(test1) gid=123 groups=123,456(test2)
$ tail -1 error_log

tail: error_log: Permission denied

$ touch x
$ ls -l x

-rw-rw-r--  1 test1  test1  0 Jul 25 03:54 x

$ chgrp test2 x

chgrp: x: Operation not permitted


This is just NFSv3.  The FreeBSD client mounts with only the nosuid
option.  The Linux server's export options are
rw,async,no_subtree_check.

Does anyone what's going on here?  The same test works fine with
FreeBSD<->FreeBSD and Linux<->Linux.  Unfortunately I can't figure out
if this is on the FreeBSD side, the Linux side, or (most likely) just
a config error on my part.

Thanks for any pointers!