Incorrect result (missing row) with UNION DISTINCT in anchor parts

create table t1(a int);

insert into t1 values(1),(2);

insert into t1 values(1),(2);

set @c=0, @d=0;

WITH RECURSIVE qn AS

(

select 1,0 as col from t1

union distinct

select 1,0 from t1

union all

select 3, 0*(@c:=@c+1) from qn where @c<1

union all

select 3, 0*(@d:=@d+1) from qn where @d<1

)

select * from qn;

MariaDB [test]> create table t1(a int);

Query OK, 0 rows affected (0.35 sec)

MariaDB [test]> insert into t1 values(1),(2);

Query OK, 2 rows affected (0.15 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> insert into t1 values(1),(2);

Query OK, 2 rows affected (0.06 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> set @c=0, @d=0;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> WITH RECURSIVE qn AS

    -> (

    -> select 1,0 as col from t1

    -> union distinct

    -> select 1,0 from t1

    -> union all

    -> select 3, 0*(@c:=@c+1) from qn where @c<1

    -> union all

    -> select 3, 0*(@d:=@d+1) from qn where @d<1

    -> )

    -> select * from qn;

+---+-----+

| 1 | col |

+---+-----+

| 1 |   0 |

| 3 |   0 |

+---+-----+

2 rows in set (0.00 sec)

MariaDB [test]> select 1,0 as col from t1         # expected result

    -> union distinct

    -> select 1,0 from t1

    -> union all

    -> select 3, 0 

    -> union all

    -> select 3, 0;

+---+-----+

| 1 | col |

+---+-----+

| 1 |   0 |

| 3 |   0 |

| 3 |   0 |

+---+-----+

3 rows in set (0.00 sec)