CREATE VIEW with `ALGORITHM=MERGE` and `ALGORITHM=TEMPTABLE` return different grouped `VAR_SAMP(...)` results under `HAVING ... IS NULL`

Two views with the same definition but different view algorithms produce different results on MariaDB.
The query groups by an indexed column, computes `VAR_SAMP(...)`, and filters with `HAVING ... IS NULL`.
`TEMPTABLE` returns the expected `NULL` group, while `MERGE` incorrectly returns a non-`NULL` aggregate value.

How to repeat：

CREATE TABLE t (

  a INT,

  b CHAR(1),

  KEY (b)

);

INSERT INTO t VALUES

  (1, 'a'),

  (2, 'a'),

  (3, 'b');

CREATE ALGORITHM=MERGE VIEW vm AS

SELECT *

FROM t;

CREATE ALGORITHM=TEMPTABLE VIEW vt AS

SELECT *

FROM t;

SELECT VAR_SAMP(a) AS v

FROM vm

GROUP BY b

HAVING v IS NULL;

SELECT VAR_SAMP(a) AS v

FROM vt

GROUP BY b

HAVING v IS NULL;

Observed result - Query A (`MERGE`):

Observed result - Query B (`TEMPTABLE`):