Query based major compaction always creates only one bucket file

set hive.execution.engine=mr;
drop table if exists tbl2;
create table tbl2 (a int, b int) clustered by (a) into 2 buckets stored as ORC TBLPROPERTIES('bucketing_version'='2', 'transactional'='true', 'compactorthreshold.hive.compactor.delta.num.threshold'='3');
insert into tbl2 values(1,2),(1,3),(1,4),(2,2),(2,3),(2,4);
insert into tbl2 values(3,2),(3,3),(3,4),(4,2),(4,3),(4,4);
delete from tbl2 where b = 2;
insert into tbl2 values(5,2),(5,3),(5,4),(6,2),(6,3),(6,4);
delete from tbl2 where a = 1;