Automatically Not Null when adding a DEFAULT Clause

Today I learned something new about the DEFAULT ON NULL clause.
Since Oracle database release 12 it is possible to define a DEFAULT ON NULL clause which is used when a NULL is provided as value. The thing I didn't know is that the column is automatically defined as NOT NULL.
Is this documented behaviour? Yes, it is. Quote from the documentation:

When you specify ON NULL, the NOT NULL constraint and NOT DEFERRABLE constraint state are implicitly specified.

SQL> select banner
  2    from v$version
  3  /

BANNER
--------------------------------------------------------------------------------
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production

SQL> create table t 
  2  (col1 varchar2(10) default on null 'hello')
  3  /

Table T created.

SQL> desc t
Name Null?    Type         
---- -------- ------------ 
COL1 NOT NULL VARCHAR2(10) 

And in case you are wondering if you can change the column afterwards to NULL, the answer is "no".

SQL> alter table t 
  2  modify (col1 null)
  3  /

Error starting at line : 1 in command -
alter table t 
modify (col1 null)
Error report -
ORA-01451: column to be modified to NULL cannot be modified to NULL
01451. 00000 -  "column to be modified to NULL cannot be modified to NULL"
*Cause:    the column may already allow NULL values, the NOT NULL constraint
           is part of a primary key or check constraint.
*Action:   if a primary key or check constraint is enforcing the NOT NULL
           constraint, then drop that constraint.