Testing using
% smbclient -L //samba-2 -U test
I get logs entries below.
(Sun Apr 23 17:36:52 2017) [sssd[nss]] [nss_cmd_getpwnam_search] (0x0100): Requesting info
for [samba-2\nobody(a)CORP.CELADONSYSTEMS.COM]
(Sun Apr 23 17:36:52 2017) [sssd[nss]] [sysdb_search_user_by_upn] (0x0040): Error: 5
(Input/output error)
(Sun Apr 23 17:36:52 2017) [sssd[nss]] [nss_cmd_getpwnam_search] (0x0040):
sysdb_search_user_by_upn failed.
(Sun Apr 23 17:36:52 2017) [sssd[nss]] [client_recv] (0x0200): Client disconnected!
Not sure it's relevant, but posting it here anyways.