assertion failed with assertion: NotExist when updating a masterialized table

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
create table t(a int);
create materialized view log on t (a);
insert into t values (1);
select _tidb_rowid, a from t;
select _tidb_rowid, a, `_MLOG$_DML_TYPE`, `_MLOG$_OLD_NEW` from `$mlog$t`;
update t set a = a+1 where a = 1;
insert into t values (2), (3);
```

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

no error

### 3. What did you see instead (Required)

an error reported:

```
> insert into t values (2), (3);
(HY000): assertion failed: key: 7480000000000086535f728000000000000003, assertion: NotExist, start_ts: 464245659724677120, existing start ts: 464245659724414977, existing commit ts: 464245659724414978
```

### 4. What is your TiDB version? (Required)

feature/release-8.5-materialized-view

No files or tests are named. Start by running the provided SQL reproduction on the feature/release-8.5-materialized-view version, focusing on the update followed by the multi-row insert and the assertion details. Done means the sequence completes without the NotExist assertion failure.

Written by the indexing model from the issue text.