FreeBSD NFS client sends only 15 supplemental groups

While looking into another issue related to users with more than 16
groups and AUTH_SYS, I noticed that my FreeBSD client was denied access
whereas linux and illumos clients with the same user worked fine.

There is a fix in illumos named "Support for more than 16 groups with
AUTH_SYS" (https://www.illumos.org/issues/5296) where if incoming
request has NGRPS (16) groups, mountd would lookup all additional groups
for uid and pass it back to kernel.

The user in question (it's just a test scenario so pretty generic):

$ id someuser
uid=30001(someuser) gid=30001(somegroup)
groups=30001(somegroup),20001(group01),20002(group02),20003(group03),
20004(group04),20005(group05),20006(group06),20007(group07),20008(group08),
20009(group09),20010(group10),20011(group11),20012(group12),20013(group13),
20014(group14),20015(group15),20016(group16),20017(group17),20018(group18),
20019(group19),20020(group20),20021(group21),20022(group22),20023(group23),
20024(group24),20025(group25),20026(group26),20027(group27),20028(group28),
20029(group29),20030(group30),20031(group31)

This works for illumos (solaris) and linux clients:

RPC:  ----- SUN RPC Header -----
RPC:
RPC:  Record Mark: last fragment, length = 168
RPC:  Transaction id = 2395676659
RPC:  Type = 0 (Call)
RPC:  RPC version = 2
RPC:  Program = 100003 (NFS), version = 3, procedure = 1
RPC:  Credentials: Flavor = 1 (Unix), len = 92 bytes
RPC:     Time = 0
RPC:     Hostname = xxx
RPC:     Uid = 30001, Gid = 30001
RPC:     Groups = 20001 20002 20003 20004 20005 20006 20007 20008 20009
20010 20011 20012 20013 20014 20015 20016
RPC:  Verifier   : Flavor = 0 (None), len = 0 bytes
RPC:

And does not work for FreeBSD client:

RPC:  ----- SUN RPC Header -----
RPC:
RPC:  Record Mark: last fragment, length = 176
RPC:  Transaction id = 3289650253
RPC:  Type = 0 (Call)
RPC:  RPC version = 2
RPC:  Program = 100003 (NFS), version = 3, procedure = 4
RPC:  Credentials: Flavor = 1 (Unix), len = 96 bytes
RPC:     Time = 23-Jan-26 09:55:43
RPC:     Hostname = xxx
RPC:     Uid = 30001, Gid = 30001
RPC:     Groups = 20001 20002 20003 20004 20005 20006 20007 20008 20009
20010 20011 20012 20013 20014 20015
RPC:  Verifier   : Flavor = 0 (None), len = 0 bytes
RPC:

Note the number of groups - it's 16 (20001 through 20016) for the former
and 15 (20001 through 20015) for the latter.  Now the code in illumos's
mountd does the "ngids == NGRPS" check and obviously this fails for
FreeBSD client.

I am not really familiar with NFS protocol so my question is if this is
a bug in FreeBSD client or just something undocumented in protocol so
I'll have to fix it in illumos' mountd to have both 15 and 16
supplemental groups cases?