Wrong result when rebuilding MV with count(col) incrementally

create table t1(a char(15), b int) stored as orc TBLPROPERTIES ('transactional'='true');

insert into t1(a, b) values ('old', 1);

create materialized view mat1 stored as orc TBLPROPERTIES ('transactional'='true') as
select t1.a, count(t1.b), count(*) from t1 group by t1.a;

delete from t1 where b = 1;

insert into t1(a,b) values
('new', null);

alter materialized view mat1 rebuild;

select * from mat1;

new            	1	1

new            	0	1