Index: samba-4.5.2+dfsg/source3/auth/auth_util.c
===================================================================
--- samba-4.5.2+dfsg.orig/source3/auth/auth_util.c
+++ samba-4.5.2+dfsg/source3/auth/auth_util.c
@@ -109,7 +109,8 @@ NTSTATUS make_user_info_map(TALLOC_CTX *
 	char *internal_username = NULL;
 	bool upn_form = false;
 
-	if (client_domain[0] == '\0' && strchr(smb_name, '@')) {
+	if (client_domain[0] == '\0' && strchr(smb_name, '@') &&
+	    lp_server_role() != ROLE_STANDALONE) {
 		upn_form = true;
 	}