[Samba] Posix vs. Windows File/Directory Permissions

[Mike]

I want to get a better understanding of what's happening between the posix
permissions and windows permissions.

I start with a Samba 4 AD DC with a share:  /mnt/data

All subfolders and files have permissions and ownership that look like:

drwxrwxrwx. 539 root root  52K May 12 17:50

The Samba 4 Admin and User accounts are not local users on the linux
server; they are only AD domain accounts.  The /mnt/data share is set read
only = no.  I thought the AD DC ignores the posix ownership and permission
settings, and they are controlled by ADUC settings via windows tools.  But
when I chmod the directory to 0770:

drwxrwx---  539  root  root  52K May 12  17:53

I can no longer access the share /mnt/data using AD DC Admin or User
accounts.

Can I get an RTFM pointer that addresses what happening in this example?
Thanks for your help.

Mike