acl directory_inherit and file_inherit do not always work

Our Macs are set up with several user accounts. Each user needs read and write access to all of the files in several shared folders. All users are part of a Group, "multicen". Each of the shared folders is set up with an ACL:

> ls -el

drwxr-xr-x+ 6 lbotti admin 192 Oct 2 20:26 Site_101

0: group:multicen inherited allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit

Depending on how subfolders and files are created within one of these shared folders, sometimes they inherit the ACL properly and sometimes they don't. For example, using Finder to drag a folder into one of these shared folders results in that folder NOT inheriting the ACL.

What's going on and how can we fix this?