I'm having the same issue running the latest beta. It creates an account in AD but still says not joined. Nothing special in AD. Just 1 2012 server running AD and DNS.
Dec 21 10:35:35 NAS emhttpd: shcmd (4239): rm -f /etc/avahi/services/smb.service
Dec 21 10:35:35 NAS avahi-daemon[4063]: Files changed, reloading.
Dec 21 10:35:35 NAS avahi-daemon[4063]: Service group file /services/smb.service vanished, removing services.
Dec 21 10:35:35 NAS emhttpd: shcmd (4242): /etc/rc.d/rc.nfsd stop
Dec 21 10:35:35 NAS rpc.mountd[18186]: Caught signal 15, un-registering and exiting.
Dec 21 10:35:36 NAS kernel: nfsd: last server has exited, flushing export cache
Dec 21 10:35:36 NAS emhttpd: Starting services...
Dec 21 10:35:36 NAS emhttpd: shcmd (4246): /etc/rc.d/rc.samba restart
Dec 21 10:35:38 NAS root: Starting Samba: /usr/sbin/smbd -D
Dec 21 10:35:38 NAS root: /usr/sbin/nmbd -D
Dec 21 10:35:38 NAS smbd[26919]: [2020/12/21 10:35:38.781779, 0] ../../source3/auth/auth_util.c:1397(make_new_session_info_guest)
Dec 21 10:35:38 NAS smbd[26919]: create_local_token failed: NT_STATUS_INVALID_PARAMETER_MIX
Dec 21 10:35:38 NAS smbd[26919]: [2020/12/21 10:35:38.781859, 0] ../../source3/smbd/server.c:2042(main)
Dec 21 10:35:38 NAS smbd[26919]: ERROR: failed to setup guest info.
Dec 21 10:35:38 NAS root: /usr/sbin/wsdd
Dec 21 10:35:38 NAS nmbd[26924]: [2020/12/21 10:35:38.806762, 0] ../../lib/util/become_daemon.c:135(daemon_ready)
Dec 21 10:35:38 NAS nmbd[26924]: daemon_ready: daemon 'nmbd' finished starting up and ready to serve connections
Dec 21 10:35:38 NAS root: /usr/sbin/winbindd -D
Dec 21 10:35:38 NAS winbindd[26934]: [2020/12/21 10:35:38.884517, 0] ../../source3/winbindd/winbindd_cache.c:3203(initialize_winbindd_cache)
Dec 21 10:35:38 NAS winbindd[26934]: initialize_winbindd_cache: clearing cache and re-creating with version number 2
Dec 21 10:35:38 NAS winbindd[26934]: [2020/12/21 10:35:38.885046, 0] ../../source3/winbindd/winbindd_util.c:1289(init_domain_list)
Dec 21 10:35:38 NAS winbindd[26934]: Could not fetch our SID - did we join?
Dec 21 10:35:38 NAS winbindd[26934]: [2020/12/21 10:35:38.885080, 0] ../../source3/winbindd/winbindd.c:1466(winbindd_register_handlers)
Dec 21 10:35:38 NAS winbindd[26934]: unable to initialize domain list
Dec 21 10:35:38 NAS avahi-daemon[4063]: Files changed, reloading.
Dec 21 10:35:38 NAS avahi-daemon[4063]: Loading service file /services/smb.service.
Dec 21 10:35:38 NAS emhttpd: shcmd (4250): /etc/rc.d/rc.nfsd start
Dec 21 10:35:38 NAS root: Starting NFS server daemons:
Dec 21 10:35:38 NAS root: /usr/sbin/exportfs -r
Dec 21 10:35:38 NAS root: /usr/sbin/rpc.nfsd 8
Dec 21 10:35:38 NAS root: /usr/sbin/rpc.mountd
Dec 21 10:35:39 NAS rpc.mountd[26964]: Version 2.1.1 starting
Dec 21 10:35:39 NAS avahi-daemon[4063]: Service "NAS" (/services/smb.service) successfully established.
Dec 21 10:36:01 NAS nmbd[26924]: [2020/12/21 10:36:01.847586, 0] ../../source3/nmbd/nmbd_become_lmb.c:397(become_local_master_stage2)
Dec 21 10:36:01 NAS nmbd[26924]: *****
Dec 21 10:36:01 NAS nmbd[26924]:
Dec 21 10:36:01 NAS nmbd[26924]: Samba name server NAS is now a local master browser for workgroup MIKED on subnet 192.168.1.12
Dec 21 10:36:01 NAS nmbd[26924]:
Dec 21 10:36:01 NAS nmbd[26924]: *****
Dec 21 10:36:01 NAS nmbd[26924]: [2020/12/21 10:36:01.847742, 0] ../../source3/nmbd/nmbd_become_lmb.c:397(become_local_master_stage2)
Dec 21 10:36:01 NAS nmbd[26924]: *****
Dec 21 10:36:01 NAS nmbd[26924]:
Dec 21 10:36:01 NAS nmbd[26924]: Samba name server NAS is now a local master browser for workgroup MIKED on subnet 192.168.2.12
Dec 21 10:36:01 NAS nmbd[26924]:
Dec 21 10:36:01 NAS nmbd[26924]: *****