Looks like a version bug
19c
====
SQL>
SQL> select banner from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
SQL>
SQL> create table t
2 ( d date,
3 id int,
4 data varchar2(20)
5 )
6 partition by range (d )
7 (
8 partition p1 values less than ( date '2018-01-01' ),
9 partition p2 values less than ( date '2019-01-01' ),
10 partition p3 values less than ( date '2020-01-01' ),
11 partition p4 values less than ( date '2021-01-01' )
12 );
Table created.
SQL>
SQL> insert into t
2 select date '2017-01-01' + rownum, rownum, 'data'||rownum
3 from dual connect by level <= date '2021-01-01' - date '2017-01-01' - 1;
1460 rows created.
SQL>
SQL> alter table t modify partition p1 read only;
Table altered.
SQL>
SQL> alter table t modify partition p2 read only;
Table altered.
SQL>
SQL>
SQL> create or replace
2 view VW as
3 select d,id from t
4 where d > date '2020-01-01';
View created.
SQL>
SQL> delete from VW where id = 1100;
1 row deleted.
SQL>
SQL> insert into VW values ( sysdate, 100);
1 row created.
SQL>
SQL>
18c
====
SQL>
SQL> select banner from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production
SQL>
SQL> create table t
2 ( d date,
3 id int,
4 data varchar2(20)
5 )
6 partition by range (d )
7 (
8 partition p1 values less than ( date '2018-01-01' ),
9 partition p2 values less than ( date '2019-01-01' ),
10 partition p3 values less than ( date '2020-01-01' ),
11 partition p4 values less than ( date '2021-01-01' )
12 );
Table created.
SQL>
SQL> insert into t
2 select date '2017-01-01' + rownum, rownum, 'data'||rownum
3 from dual connect by level <= date '2021-01-01' - date '2017-01-01' - 1;
1460 rows created.
SQL>
SQL> alter table t modify partition p1 read only;
Table altered.
SQL>
SQL> alter table t modify partition p2 read only;
Table altered.
SQL>
SQL>
SQL> create or replace
2 view VW as
3 select d,id from t
4 where d > date '2020-01-01';
View created.
SQL>
SQL> delete from VW where id = 1100;
1 row deleted.
SQL>
SQL> insert into VW values ( sysdate, 100);
insert into VW values ( sysdate, 100)
*
ERROR at line 1:
ORA-14466: Data in a read-only partition or subpartition cannot be modified.
Have a chat to Support - there might be patches available