TiFlash data inconsistent with TiKV after modifying a column from NOT NULL to NULL

[JaySon-Huang]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

From tidb issue pingcap/tidb#61681

DROP DATABASE IF EXISTS db_1483421321;
CREATE DATABASE db_1483421321;
USE db_1483421321;
CREATE TABLE t0(c0 TINYINT, handle INT NOT NULL AUTO_INCREMENT, PRIMARY KEY(handle));
ALTER TABLE t0 SET TIFLASH REPLICA 1;
-- this SLEEP will make sure the case always reproducable, it not, make it longer
select sleep(5);
SELECT /*+ read_from_storage(tiflash[t0]) */ * FROM t0 order by 1;
SELECT /*+ read_from_storage(tikv[t0]) */ * FROM t0 order by 1;

ALTER TABLE t0 ADD COLUMN c1 DECIMAL NULL;
ALTER TABLE t0 ADD COLUMN c2 FLOAT NULL DEFAULT 0.0795439693286002;
ALTER TABLE t0 ADD COLUMN c5 DECIMAL NOT NULL;
ALTER TABLE t0 ADD COLUMN c4 DECIMAL DEFAULT 247262911;
ALTER TABLE t0 MODIFY COLUMN c2 FLOAT NOT NULL;
ALTER TABLE t0 MODIFY COLUMN c5 DECIMAL NULL;
ALTER TABLE t0 MODIFY COLUMN c1 BIGINT DEFAULT -56083770 NOT NULL;
-- first insert
INSERT IGNORE INTO t0 (c1, c2, c5, c4) VALUES (-2051270087, 0.44141045099028775, 0.0, 15523);
UPDATE t0 SET c5 = 870337888;

ALTER TABLE t0 MODIFY COLUMN c1 BIGINT NULL;
-- second insert
INSERT INTO t0 (c2, c5, c4) VALUES (0.004406799693866592, 0.8752311290235516, 14652);

2. What did you expect to see? (Required)

tiflash data is consistent with tikv

3. What did you see instead (Required)

-- in tiflash, handle=2, c1=0
SELECT /*+ read_from_storage(tiflash[t0]) */ * FROM t0 order by 1;
+--------+--------+-------------+--------------+-----------+-------+
| c0     | handle | c1          | c2           | c5        | c4    |
+--------+--------+-------------+--------------+-----------+-------+
| <null> | 1      | -2051270087 | 0.44141045   | 870337888 | 15523 |
| <null> | 2      | 0           | 0.0044067996 | 1         | 14652 |
+--------+--------+-------------+--------------+-----------+-------+

-- in tikv, handle=2, c1=null
SELECT /*+ read_from_storage(tikv[t0]) */ * FROM t0 order by 1;
+--------+--------+-------------+--------------+-----------+-------+
| c0     | handle | c1          | c2           | c5        | c4    |
+--------+--------+-------------+--------------+-----------+-------+
| <null> | 1      | -2051270087 | 0.44141045   | 870337888 | 15523 |
| <null> | 2      | <null>      | 0.0044067996 | 1         | 14652 |
+--------+--------+-------------+--------------+-----------+-------+

4. What is your TiFlash version? (Required)

v8.5.2 as the tidb issue described
v8.5.4-nextgen.202510.5