pg12 partitions question

Hey,
I got the following partitions structure in pg12 beta 3 version :

postgres=# \d+ students
Partitioned table "public.students"
Column | Type | Collation | Nullable | Default | Storage | Stats
target | Description
--------+---------+-----------+----------+---------+----------+--------------+-------------
id | integer | | not null | | plain |
|
name | text | | | | extended |
|
class | integer | | | | plain |
|
Partition key: HASH (id)
Indexes:
"students_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
"students_class_fkey" FOREIGN KEY (class) REFERENCES class(id)
Partitions: students_00 FOR VALUES WITH (modulus 20, remainder 0),
students_01 FOR VALUES WITH (modulus 20, remainder 1),
students_02 FOR VALUES WITH (modulus 20, remainder 2),
students_03 FOR VALUES WITH (modulus 20, remainder 3),
students_04 FOR VALUES WITH (modulus 20, remainder 4)

postgres=# insert into students values(20,'a',1);
ERROR: no partition of relation "students" found for row
DETAIL: Partition key of the failing row contains (id) = (20).

I'm trying to insert a few rows but some of them fail on the following
error : no partition of relation "sutdents" found for row ...

for example :
postgres=# insert into students values(20,'a',1);
ERROR: no partition of relation "students" found for row
DETAIL: Partition key of the failing row contains (id) = (20).
postgres=# insert into students values(2,'a',1);
ERROR: no partition of relation "students" found for row
DETAIL: Partition key of the failing row contains (id) = (2).
postgres=# insert into students values(1,'a',1);
ERROR: duplicate key value violates unique constraint "students_00_pkey"
DETAIL: Key (id)=(1) already exists.
postgres=# insert into students values(2,'a',1);
ERROR: no partition of relation "students" found for row
DETAIL: Partition key of the failing row contains (id) = (2).
postgres=# insert into students values(3,'a',1);
INSERT 0 1
postgres=# insert into students values(4,'a',1);
ERROR: no partition of relation "students" found for row
DETAIL: Partition key of the failing row contains (id) = (4).

The current content of the table :
postgres=# select * from students;
id | name | class
----+------+-------
1 | a | 1
3 | a | 1
(2 rows)

what am I missing ?