It doesn't work for me...
CREATE TABLE MY_TABLE (
ID NUMBER(19, 0) GENERATED BY DEFAULT ON NULL AS IDENTITY PRIMARY KEY,
name NVARCHAR2(255) NOT NULL
);
INSERT INTO MY_TABLE (ID, name) VALUES (1, 'test1');
INSERT INTO MY_TABLE (name) VALUES ('test2');
ORA-00001: unique constraint (CHRIS.SYS_C0017753) violated
select * from my_table;
ID NAME
1 test1