I briefly raised this issue months ago and am trying to resolve it again 
now.

What I am trying to do is setup permissions so multiple users on one 
machine can share full control over a particular directory hierarchy.

On Linux I have usually been able to make things work with:

    $ mkdir shared_dir
    $ chgrp shared_group shared_dir
    $ chmod g+ws shared_dir
    $ umask 2

User shells are configured with umask 2 so files they create have group 
write. Users belong to shared_group. Files and subdirs created under 
shared_dir are all in group shared_group. Files moved in retain their 
original group, but the group members still have permission to rename or 
delete them.

The problem:

$ chmod g+ws fails to set the 's' bit, and the resulting icacls output 
does not contain any "NULL SID" entries. I am seeing the same problem on 
(at least) two different systems setup by my organization. One of these 
was just re-imaged and I installed Cygwin yesterday with no customized 
configurations. AV is Windows Defender, but I suspect if that were the 
culprit, there would have been more noise.

I suspect there might be a group policy or something that is interfering 
with Cygwin's strategy for implementing POSIX permissions. I am pretty 
sure this worked correctly at some point in the past.

Has anyone encountered this?

Does group policy seem like a likely suspect? Anyone know which 
policy(ies)? I think I might be able to get IT to cut me slack if I knew 
what to ask for.

I have also played with using setfacl directly to add permissions, but 
as anyone who has read about Cygwin file permissions might guess, that 
tends to have mixed/poor results, but I'd be open to any suggestions.