This likely is also a problem in Samba 4.*, but I am not in a position to test that.
We are running Samba on a Solaris 10 system. When creating directories (folders) and files in a directory with a default ACL set up, the default ACL is not used.

For instance, for a share set up with 'inherit acls = yes' and an ACL as follows:
# file: .
# owner: schulz
# group: users
user::rwx
group::rwx              #effective:rwx
mask:rwx
other:r-x
default:user::rwx
default:group::rwx
default:mask:rwx
default:other:r-x

A new folder created through Samba ends up with an ACL as follows:
# file: foom4
# owner: schulz
# group: users
user::rwx
group::r-x              #effective:r-x
mask:r-x
other:r-x
default:user::rwx
default:group::rwx
default:mask:rwx
default:other:r-x

And permissions drwxr-sr-x+

Note that the read permission is not set for group. I can get the behavior I need in this case by adding 'inherit permissions = yes' to the share, but that only works if the parent directory has the same permissions as that desired for the new directory.