group ownership of files doesn't seem to be working quite correctly on 5.1B?

In fact, group ownership of newly created files doesn't seem to work  
at all.  I can't believe it's taken me 5 years to notice this one...

Changing a shell's group does not seem to affect the ownership of new  
files created once the group has been changed:

13:57:11 tjrc@pcs3g:/tmp$ id
uid=10787(tjrc) gid=10(staff) groups=0(system),706(ssg),558(intweb), 
555(w3adm),102(informatics)

13:57:16 tjrc@pcs3g:/tmp$ ls -l foo
ls: foo not found

13:57:37 tjrc@pcs3g:/tmp$ newgrp ssg

13:57:43 tjrc@pcs3g:~$ cd /tmp
13:57:45 tjrc@pcs3g:/tmp$ id
uid=10787(tjrc) gid=706(ssg) groups=0(system),706(ssg),558(intweb),555 
(w3adm),102(informatics)

So the gid has changed correctly, but

13:57:51 tjrc@pcs3g:/tmp$ /bin/touch foo
13:57:58 tjrc@pcs3g:/tmp$ ls -l foo
-rw-r--r--   1 tjrc     system         0 Jan 15 13:57 foo
13:58:00 tjrc@pcs3g:/tmp$

Not only has it not created the file with the group ownership that's  
my primary group, or the group to which I have changed, but it seems  
to have chosen the first group of which I am a member (0).

Now, only the system group is actually in the /etc/group file, the  
others are all NIS groups.

I thought it might be the case that newgrp(1) only works with groups  
which are present in the /etc/group file, so I edited the group file  
to make a local copy of the ssg group, and the issue persists.

I'm aware of the historical bug that affected people with groups  
split over multiple lines in /etc/group, but that isn't the case here.

But then I went back ignoring newgrp at all, just checked what  
ownership my files are being created with, and lo, it's "system" not  
my primary group of "staff".  What?!

Does anyone know what's going on?  Is this some strange configuration  
option of AdvFS that we've got wrong?  It seems likely it's actually  
at the filesystem level, since the problem also affects Linux NFS  
clients, iff the file server is running Tru64.

Can anyone help me out here?

Tim