diff -urN mantisbt-1.3.0.orig/api/soap/mc_filter_api.php mantisbt-1.3.0/api/soap/mc_filter_api.php
--- mantisbt-1.3.0.orig/api/soap/mc_filter_api.php	2016-07-10 08:09:51.000000000 +0900
+++ mantisbt-1.3.0/api/soap/mc_filter_api.php	2016-07-30 22:17:26.638837876 +0900
@@ -272,6 +272,10 @@
         }
     }
 
+    if( is_array ( $t_filter['tag_string']) ) {
+        $t_filter['tag_string'] = implode( config_get( 'tag_separator' ), $t_filter['tag_string']);
+    }
+
     $t_filter = filter_ensure_valid_filter( $t_filter );
 
     $t_page_number = $p_page_number < 1 ? 1 : $p_page_number;