Response from Cygwin list:
> $ ls -la /cygdrive/z/tmp/test-epackage.el
> -rwxr--r-- 1 ???????? ???????? 437 Dec 9 20:02
/cygdrive/z/tmp/test-epackage.el
It's not a bug.
If you use winbind and the user accounts are correctly mapped to
Windows accounts, then you would see the Cygwin UIDs/GIDs
correspoding to the SID of the AD user account.
If you don't do that, there's only an invisible mapping from the
Windows SID to the Unix uid/gid. The actual UNIX account has not
the same mapping back to the Windows SID. Instead, the SID
returned from Samba to Windows is a fake SID S-1-22-1-UnixUID or
S-1-22-2-UnixGID. (..)
The rest goes into details for setting 1:1 UID, GID mapping.