TO_DATE() returns NULL for a valid format after processing a NULL format row

When TO_DATE() evaluates a format stored in a column, a NULL format affects the following row: the next valid format returns NULL in the same scan, although it returns the correct date when queried alone.

This occurs when TO_DATE() evaluates a row-varying format expression across a valid/NULL/valid sequence.

The behavior was recorded on MariaDB 12.3.3. This equivalent reproducer demonstrates the same valid/NULL/valid sequence:

CREATE DATABASE dates_test;

USE dates_test;

CREATE TABLE formats (id INT PRIMARY KEY, f VARCHAR(20));

INSERT INTO formats VALUES (1,'YYYY-MM-DD'),(2,NULL),(3,'YYYY-MM-DD');

SELECT id, TO_DATE('2024-06-15',f) AS actual, IF(f IS NULL,NULL,CAST('2024-06-15' AS DATETIME)) AS expected FROM formats FORCE INDEX(PRIMARY) ORDER BY id;

SELECT id, TO_DATE('2024-06-15',f) AS actual FROM formats WHERE id=3;

The recorded scan returned:

id  actual                      expected

1   2024-06-15 00:00:00.000000 2024-06-15 00:00:00

2   NULL                        NULL

3   NULL                        2024-06-15 00:00:00

The recorded query selecting row 3 alone returned:

id  actual

3   2024-06-15 00:00:00.000000

Rows 1 and 3 contain the same valid format, so both should return the parsed date. Row 2 should return NULL. The isolated query confirms that row 3's input is valid; its result should not depend on whether a preceding row supplied a NULL format.