Inconsistent behaviors of UPDATE under RU & RC isolation level

Isolation Level: Read Uncommitted & Read Committed.
The behaviors of UPDATE for different rows are inconsistent. Just some of the rows are not updated while others are.

/* init */ drop table if exists t;

/* init */ create table t(a int, b int);

/* init */ insert into t values(null, 1), (2, 2), (null, null), (null, 3), (4, null);

/* s1 */ begin;

/* s1 */ update t set a = 10 where 1;

/* s2 */ begin;

/* s2 */ update t set b = 20 where a; -- blocked

/* s1 */ commit; -- s2 unblocked

/* s2 */ commit;

select * from t;

+------+------+

| a    | b    |

+------+------+

|   10 |    1 |

|   10 |   20 |

|   10 |   20 |

|   10 |   20 |

|   10 |   20 |

+------+------+

The field b of the first row is not updated but other rows are updated.

After several attempts, I found that the results depend on the distribution of the data in the table rather than the timing.

If the initial table is like:

+------+------+

| a    | b    |

+------+------+

|    1 |    1 |

|    2 |    2 |

| null | null |

| null |    3 |

|    4 | null |

+------+------+

then, the result will be

+------+------+

| a    | b    |

+------+------+

|   10 |   20 |

|   10 |   20 |

|   10 |   20 |

|   10 |   20 |

|   10 |   20 |

+------+------+

More specifically, if there exists a row whose column `a` is not NULL, and it is the first record from top to bottom whose column `a` is not NULL. Then, all rows before it with a NULL column `a` will not be updated, while all rows after it with a NULL column `a` will be updated. This means that even the execution of a single statement is not atomic.

This bug can be reproduced under Read Uncommitted and Read Committed.