Oracle SUBSTR results in longer column?

It seems that SUBSTR can result in a longer column? What's going on here?

CREATE TABLE t (
    long_field VARCHAR2(1024)
);

SELECT
    column_name,
    data_length
FROM
    all_tab_cols
WHERE
    table_name = 'T';

INSERT INTO t ( long_field ) VALUES ( 'abcdef' );

CREATE TABLE t2
    AS
        SELECT
            substr(long_field, instr(long_field, 'c') + 1) short_field
        FROM
            t;

SELECT
    column_name,
    data_length
FROM
    all_tab_cols
WHERE
    table_name = 'T2';

I'm on Oracle 18:

select BANNER
from V$VERSION;

SUBSTR returns a VARCHAR2(4000) as that is the maximum length sub-string that could be returned from the function.

Just because your function substr(long_field, instr(long_field, 'c') + 1) returns 'def' doesn't mean that it changes the data-type to have a data-length that is the bounded by the data (i.e. VARCHAR2(3)) as SUBSTR could still (for other data) return a 4000 byte string.

If you want a lower data length then use CAST:

CREATE TABLE t3 AS
  SELECT CAST(
           substr(long_field, instr(long_field, 'c') + 1)
           AS VARCHAR2(10)
         ) short_field
  FROM   t;

Then the data dictionary will contain:

fiddle