Share definition:

[data]
        path = /var/data
        valid users = +CORP\www-writers, CORP\vova
        read only = No
        inherit acls = Yes
        inherit owner = Yes
        vfs objects = full_audit, recycle
        recycle:maxsize = 52428800
        recycle:exclude = .~lock.*
        recycle:touch_mtime = yes
        recycle:touch = no
        recycle:versions = yes
        recycle:keeptree = yes
        recycle:directory_mode = 0700
        recycle:repository = /mnt/data/samba/recycle/%u
        full_audit:prefix = share=%S|id=%u|ip=%I
        full_audit:priority = INFO
        full_audit:facility = LOCAL2
        full_audit:failure = unlink rmdir mkdir write rename aio_write pwrite connect disconnect
        full_audit:success = unlink rmdir mkdir write rename aio_write pwrite connect disconnect

ACL on folder:
# file: test
# owner: wwwadm
# group: www
user::rwx
user:vova:rwx
group::---
group:CORP\134www-writers:rwx
mask::rwx
other::---
default:user::rwx
default:user:vova:rwx
default:group::---
default:group:CORP\134www-writers:rwx
default:mask::rwx
default:other::---

So group CORP\www-writers always should have all rights on created files. This definition worked before update to 3.5.8 (in 3.5.6, 3.5.7 for example). In 3.5.8 mask doesn't inherit - it becomes "r--".