Use global default_notify if flag not in DB config

The web UI currently doesn't support the explicit notify flag,
hence, when default_notify_flag and notify_flag are saved based
on administrator using web UI to override configs, such settings
will not include the 'explicit' flag causing it to be considered
false.  The fix is to fallback to the global config when overridden
configs in db for notify_flags and default_notify_flags don't have
the requested flag.

Fixes #20371

core/email_api.php

@@ -215,24 +215,36 @@ function email_ensure_not_disposable( $p_email ) {
 }
 
 /**
- * email_notify_flag
  * Get the value associated with the specific action and flag.
  * For example, you can get the value associated with notifying "admin"
  * on action "new", i.e. notify administrators on new bugs which can be
  * ON or OFF.
  * @param string $p_action Action.
  * @param string $p_flag   Flag.
- * @return integer
+ * @return integer 1 - enabled, 0 - disabled.
  */
 function email_notify_flag( $p_action, $p_flag ) {
+	# If flag is specified for the specific event, use that.
 	$t_notify_flags = config_get( 'notify_flags' );
-	$t_default_notify_flags = config_get( 'default_notify_flags' );
 	if( isset( $t_notify_flags[$p_action][$p_flag] ) ) {
 		return $t_notify_flags[$p_action][$p_flag];
-	} else if( isset( $t_default_notify_flags[$p_flag] ) ) {
+	}
+
+	# If not, then use the default if specified in database or global.
+	# Note that web UI may not support or specify all flags (e.g. explicit),
+	# hence, if config is retrieved from database it may not have the flag.
+	$t_default_notify_flags = config_get( 'default_notify_flags' );
+	if( isset( $t_default_notify_flags[$p_flag] ) ) {
 		return $t_default_notify_flags[$p_flag];
 	}
 
+	# If the flag is not specified so far, then force using global config which
+	# should have all flags specified.
+	$t_global_default_notify_flags = config_get_global( 'default_notify_flags' );
+	if( isset( $t_global_default_notify_flags[$p_flag] ) ) {
+		return $t_global_default_notify_flags[$p_flag];
+	}
+
 	return OFF;
 }