Wrong results when computing window functions with same sort key but different direction

create table t1 (

  pk int primary key,

  a int,

  b int,

  c char(10)

);

insert into t1 values

( 1, 0, 1, 'one'),

( 2, 0, 2, 'two'),

( 3, 0, 3, 'three'),

( 4, 1, 1, 'one'),

( 5, 1, 1, 'two'),

( 6, 1, 2, 'three'),

( 7, 2, NULL, 'n_one'),

( 8, 2, 1,    'n_two'),

( 9, 2, 2,    'n_three'),

(10, 2, 0,    'n_four'),

(11, 2, 10,   NULL);

select row_number() over (order by a), row_number() over (order by a desc) from t;

select pk,

         row_number() over (order by pk desc) as r_desc,

         row_number() over (order by pk asc) as r_asc

from t1;

pk	r_desc	r_asc

1	11	11

2	10	10

3	9	9

4	8	8

5	7	7

6	6	6

7	5	5

8	4	4

9	3	3

10	2	2

11	1	1