Query returns two rows instead of one

CREATE TABLE test (

col1 NUMBER);

insert into test(col1) values (1);

CREATE TABLE test2 (

col1 varchar2(10),

col2 varchar2(100)

);

insert into test2(col1, col2) values('1', 'Test 1');

insert into test2(col1, col2) values('01', 'Test 01');

select a.col1, (select max(b.col2) from test2 b where a.col1 = b.col1) from test a;

The select in the end returns two rows instead of one which is strange.

This appears in Oracle 12c and 18c but not in 11g.

Is this a bug in Oracle 12c and 18c?